[libc] Fix LLVM_LIBC_FUNCTION on 32-bit x86 Windows (#213456)

Previously, trying to compile libm with LIBC_COPT_PUBLIC_PACKAGING
defined failed on 32-bit Windows with

    error: alias must point to a defined variable or function

This is because 32-bit Windows adds a leading underscore to __cdecl C
functions, making the alias declaration not find its target symbol name.
The same problem exists on Apple platforms, which as solution don't emit
the alias for the C++ LIBC_NAMESPACE:: symbol.

Do the same on 32-bit Windows as on Apple platforms: Emit only
the underscore-prefixed symbol, not the LIBC_NAMESPACE:: alias.

GitOrigin-RevId: 350089d0cb1652e6ff44888f46d6487db5e696e6