[libc++] Provide c++03 alternative for va_copy if available in xlocale.h

Summary:
If we are on c++03 mode for some reason, and __builtin_va_copy is
available, then use it instead of error out on not having va_copy
in 03 mode.

Reviewed by: ldionne

Differential Revision: https://reviews.llvm.org/D100336