llvmPackages_11: 11.0.0rc5 -> 11.0.0 and various fixes/improvements:
- llvmPackages_11 now compiles on Darwin
- libcxx now compiles on Linux
- Ported #91293 to 11.0.0
- Try to anticipate #100388
compiler-rt (and as a result clang) can't be build for i686 (as noticed here: #99984).
The patch adds the required variables and should result in the same behavior as in the nixpkgs-llvm10. It essentially forces to use i386 buildins when using i486, i586 or i686, which are not supported.
Fixes#100392