Silence GCC warnings when using libc++ headers.

GCC 12 raises warnings about literal operator suffixes not preceded by
'_' in libc++ headers such as <string_view> as it doesn't recognize
libc++ headers being an implementation of the standard.

GCC 12 also warns about clang-specific pragmas in <locale>.

Disabling these warnings globally for all C++ code is not ideal, but
is a better option than patching libc++ headers to ignore these
warnings.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37530