linux: link system llvm/clang/lld may cause two distinct libc++ linkages

[mikdusan]

• when linking system llvm/clang/lld
• on linux distros with libc++ as their system choice for c++ (such as chimera linux)
• zig only searches for system libstdc++ and fallback is to use bundled libc++

libc++.so is never found and zig .cpp artifacts link to zig-bundled libc++.a, while llvm/clang/lld implicitly link to system libc++.so. This results in 2 distinct copies of (some) globals in the zig exe artifact.

chimera-linux report: chimera-linux/cports#263