[bug#49471] [PATCH] gnu: lld: Update to 12.0.0.

[terramorpha]

Hi guix. While updating my personal package definition of zig to 0.8.0, 
I had to write one for lld@12 too.
(IIUC, only llvm and clang were upgraded to 12, I couldn't find lld when 
`guix search`-ing)

Anyway, the release tarball assumes that it will be unpacked and built 
in the same directory as the libunwind source.
This is why at build time, I added a phase to patch the CMakeLists.txt 
and make it look for libunwind in a specific place.

Cheers