Add LLVM libunwind v12.0.1

[ararslan]

No description provided.

[ararslan]

[03:26:00] CMake Error at CMakeLists.txt:2 (message):
[03:26:00]   libunwind requires being built in a monorepo layout with libcxx available

UGH. This apparently wasn't the case for libunwind 11 based on #2360, #2557, and #2570. I'm guessing what will need to happen is to use LLVM_full as a BuildDependency, build libcxx first in the script, then build libunwind. But if libunwind 12 actually needs to link to libc++, I would imagine we'd need a separate libcxx build recipe that would then be a Dependency for libunwind.

[ararslan]

Wait what, llvm/llvm-project@8c03fdf I think it only needs libcxx and the monorepo so it can reuse one CMake file?!

[ararslan]

Things seem to be working now. I worked around the build system change by straight up reverting the commit that touched it. A better long-term solution would of course be to properly adjust how we build it here, buuuuuuuuuut...

[giordano]

Not super happy about reverting a commit, but might be good as a temporary solution

[ararslan]

yolo