fix: add exception for 32-bit ARM builds which need glibc 2.28

note: focal uses glibc 2.31, which is used by gitian, so our build
      environment has support for a version of glibc that far exceeds
      our maximum supported version (2.28). this could be remedied by
      switching to debian buster (oldstable) which uses 2.28.

contrib/devtools/test-symbol-check.py

@@ -30,12 +30,17 @@ def test_ELF(self):
         cc = determine_wellknown_cmd('CC', 'gcc')
 
         # there's no way to do this test for RISC-V at the moment; we build for
-        # RISC-V in a glibc 2.27 envinonment and we allow all symbols from 2.27.
+        # RISC-V in a glibc 2.31 envinonment and we allow all symbols from 2.27.
         if 'riscv' in get_machine(cc):
             self.skipTest("test not available for RISC-V")
 
+        # there's no way to do this test for ARM at the moment; we build for
+        # ARM in a glibc 2.31 envinonment and we allow all symbols from 2.28.
+        if 'arm' in get_machine(cc):
+            self.skipTest("test not available for 32-bit ARM")
+
         # nextup was introduced in GLIBC 2.24, so is newer than our supported
-        # glibc (2.17), and available in our release build environment (2.24).
+        # glibc (2.18), and available in our release build environment (2.31).
         with open(source, 'w', encoding="utf8") as f:
             f.write('''
                 #define _GNU_SOURCE