Warn that SUPPORT_BIG_ENDIAN is experimental

[sbc100]

Support for big endian targets is "best effort" and not fully supported or tested.

Also, as far as I'm aware there is only one or two users of this feature so I think it makes sense for it to stay as "best effort" for the foreseeable future.

See #25042

[juj]

I know this is well-intentioned, though this PR now disrupts running the bigendian0 test suite, where majority of the tests then fail due to warnings being treated as errors.

E.g.

$ test/runner bigendian0.test_autodebug_wasm
Running test_core: (1 tests)
test_autodebug_wasm (test_core.bigendian0.test_autodebug_wasm) ... env_modify: {'EMCC_AUTODEBUG': '1'}
emcc: error: -sSUPPORT_BIG_ENDIAN is experimental, not all features are fully supported. [-Wexperimental] [-Werror]
None
None
FAIL

======================================================================
FAIL: test_autodebug_wasm (test_core.bigendian0.test_autodebug_wasm)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 1847, in run_process
    rtn = shared.run_process(cmd, check=check, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/tools/shared.py", line 112, in run_process
    ret = subprocess.run(cmd, check=check, input=input, *args, **kw)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/emcc', '/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/core/test_autodebug.c', '-o', 'test_autodebug.js', '-sNO_DEFAULT_TO_CXX', '-sSUPPORT_BIG_ENDIAN', '-Wclosure', '-Werror', '-Wno-limited-postlink-optimizations', '-O0', '-Wno-unused-command-line-argument', '-mreference-types']' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 986, in resulting_test
    return func(self, *args)
           ^^^^^^^^^^^^^^^^^
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 720, in metafunc
    func(self, *args, **kwargs)
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/test_core.py", line 304, in decorated
    f(self, *args, **kwargs)
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 481, in modified
    return f(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/test_core.py", line 7043, in test_autodebug_wasm
    output = self.do_runf('core/test_autodebug.c', 'success')
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 2027, in do_runf
    return self._build_and_run(filename, expected_output, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 2056, in _build_and_run
    js_file = self.build(filename, **kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 1508, in build
    self.run_process(cmd, stderr=self.stderr_redirect if not DEBUG else None)
  File "/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/common.py", line 1852, in run_process
    self.fail(f'subprocess exited with non-zero return code({e.returncode}): `{shlex.join(cmd)}`')
AssertionError: subprocess exited with non-zero return code(1): `/mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/emcc /mnt/ssd1000/emscripten_buildbot/worker_root/x10dax-linux-mint22/emscripten_x10dax/build/emscripten/main/test/core/test_autodebug.c -o test_autodebug.js -sNO_DEFAULT_TO_CXX -sSUPPORT_BIG_ENDIAN -Wclosure -Werror -Wno-limited-postlink-optimizations -O0 -Wno-unused-command-line-argument -mreference-types`

----------------------------------------------------------------------
Ran 1 test in 0.504s

FAILED (failures=1)

I don't think that it would be productive to modify each test to account for this warning message being possibly present.. probably would be simplest to just revert this? (I don't think there have ever been people complaining about expectations around -sSUPPORT_BIG_ENDIAN so far?)

[sbc100]

Sorry the marking as experimental raced with the adding of the new test config. Fix is in #25090