SWT cannot yet pass the yklua test suite

[ltratt]

I thought SWT was capable of running -- though rather slowly! -- all the things that HWT can. But on a fresh clone on a debug build if I try running yklua's test suite I get:

$ cd yklua/tests
$ YKD_OPT=0 YKD_SERIALISE_COMPILATION=1 ../src/lua -e"_U=true" all.lua
...
***** FILE 'cstack.lua'*****
testing stack overflow detection
testing stack overflow in message handling
.final count:   250037
testing recursion inside pattern matching
testing stack-overflow in recursive 'gsub'
final count:    197
testing stack-overflow in recursive 'gsub' with metatables
final count:    99
testing limits in coroutines inside deep calls
final count:    196
chain of 'coroutine.close'
thread '<unnamed>' panicked at ykrt/src/compile/jitc_yk/codegen/x64/mod.rs:2012:30:
ConstPtr(0)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread '<unnamed>' panicked at ykrt/src/mt.rs:229:39:
called `Result::unwrap()` on an `Err` value: Any { .. }
fatal runtime error: failed to initiate panic, error 5
Aborted

This is immediately a bit fishy, because I would expect swt to create identical traces to hwt, but here we seem to be encountering a null pointer. However, just in case, I implemented support for constant pointers in this experimental branch. With YKD_OPT=0 this no longer panics but corrupts yklua's state leading to a Lua level backtrace:

...
***** FILE 'cstack.lua'*****
testing stack overflow detection
testing stack overflow in message handling
.final count:   250037
testing recursion inside pattern matching
testing stack-overflow in recursive 'gsub'
final count:    197
testing stack-overflow in recursive 'gsub' with metatables
final count:    99
testing limits in coroutines inside deep calls
final count:    196
chain of 'coroutine.close'
../src/lua: cstack.lua:105: attempt to call a boolean value (field 'resume')
stack traceback:
        cstack.lua:105: in main chunk
        (...tail calls...)
        all.lua:181: in main chunk
        [C]: in ?

Can it be that we never solved the unmappable problem (see e.g. #1095 or #980)? Or is there some other problem in swt? Either way, we should almost certainly fix this, because the longer we can't run swt on our test suites, the more likely that deeper harder-to-fix problems are going to creep in.

CC @ptersilie.