libcxx: Don't apply ABI tags to extern "C" fns

GCC rejects ABI tags on non mangled functions, as they would otherwise
be a no-op.  This commit replaces such instances with equivalent
_LIBCPP_HIDE_FROM_ABI constants but without ABI tags attached.

  .../include/c++/v1/__support/musl/xlocale.h:28:68: error: 'abi_tag'
  attribute applied to extern "C" declaration 'long long int
  strtoll_l(const char*, char**, int, locale_t)'
     28 | strtoll_l(const char *__nptr, char **__endptr, int __base, locale_t) {
        |                                                                    ^

Bug: https://bugs.gentoo.org/869038

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D142415