Limit use of llvm-config with USE_SYSTEM_LLVM on Darwin

[carlocab]

As noted in #29981 and #30459, llvm-config is broken on Darwin, and
sadly hasn't been fixed since then.

Building Julia on Darwin with USE_SYSTEM_LLVM doesn't currently work,
as calls to llvm-config produce the wrong flags. This PR fixes that.

For reference, llvm-config --libfiles still prints out a long string of static
libraries, and llvm-config --libs returns a long string of linker flags to
individual LLVM libraries. Attempting to use those linker flags (e.g. -lLLVMXRay)
leads to a linker failure about missing symbols.

[haampie]

This is not limited to Darwin, you get this when you build with LLVM_BUILD_LLVM_DYLIB=ON and LLVM_LINK_LLVM_DYLIB=OFF on Linux too.

Note that --system-libs is broken too :(. It just gives the linker flags -lz, -ltinfo even when LLVM is built with -L/non-standard/path/to/libz/lib -lz, -L/non-standard/path/to/ncurses/lib -ltinfo.

[vchuravy]

What happens if you use llvm-config --shared?

[carlocab]

Do you mean --link-shared? llvm-config has no --shared flag. Here's what happens:

❯ llvm-config --libfiles --link-shared
llvm-config: error: libLLVM-12.dylib is missing

That's what this snippet is for (cf. #30033). Creating the symlink works, but you don't get this symlink if you don't already know to do it for yourself when installing LLVM (or you use a package manager that does it for you).