c18n, libgcc_s: Support a c18n-aware libunwind.

[dstolfa]

These 2 commits are bundled together with CTSRD-CHERI/llvm-project#731 into #2003 for easy testing. There is currently a lot of code duplication across longjmp, resume and unsealed resume, and I'm not too sure how to eliminate it since we're in a context where we can't perform a function call, so any function would have to be always inlined (while the compiler supports this, I'm not sure if it's good practice here?). I also wanted to avoid making too much of it a macro so that debugging of these functions doesn't become painful.

[dstolfa]

Update the diff to more closely match the semantics of what we have in the demo-2024-03 branch. This temporarily works around the fact that this patch doesn't currently include ubiquitous c18n, and all that is required to support it is to remove the two #ifndefs around C18N_ENABLED.

[dstolfa]

Removed an unused argument in _rtld_unw_setcontext and _rtld_unw_setcontext_unsealed.

[dstolfa]

Update to cherry-pick a different commit from @dpgao and improve the workaround for ubiq-c18n not being in dev yet.

[dpgao]

This all looks good to me apart from some minor changes. Let's try to remove the symbol files in libgcc_s and see if CI is happy.
2a35164 can be squashed into b6fb884.

[dstolfa]

This all looks good to me apart from some minor changes. Let's try to remove the symbol files in libgcc_s and see ...

Seems like the weak symbols don't get replaced in that case (running on top of rtld-c18n):

Breakpoint 4, _rtld_unw_getcontext () at /home/ds815/cppexcept_otypes/contrib/subrepo-cheri-libunwind/src/UnwindRegistersSave.S:842
842     /home/ds815/cppexcept_otypes/contrib/subrepo-cheri-libunwind/src/UnwindRegistersSave.S: No such file or directory.
(gdb) disas
Dump of assembler code for function _rtld_unw_getcontext:
=> 0x0000000040487efc <+0>:     mov     c2, csp
   0x0000000040487f00 <+4>:     str     c2, [c1]
   0x0000000040487f04 <+8>:     ret     c30
End of assembler dump.

[dstolfa]

Fix a conflict with ubiq-c18n

[dstolfa]

Update to address @dpgao's comments.