Add check to verify libc++ is shared by LLVM/Clang

This check is needed because if static/dynamic linking is mixed incorrectly,
it's possible for Clang and LLVM to end up with duplicate "copies" of libc++.

This is not benign: Static variables are not shared, so equality comparisons
that depend on pointers to static variables will fail. One such failure is
std::generic_category(), which causes POSIX error codes to compare as unequal
when passed between LLVM and Clang.

I believe this is the cause of https://github.com/ziglang/zig/issues/11168

In order to avoid affecting build times when Zig is repeatedly invoked,
we only enable this check for "zig env" and "zig version"

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