libc++: fix compiling <locale> with -fmodules

In /usr/include/c++/v1/__locale_dir/locale_base_api.h, xlocale.h is
included without first including stdio.h and stdlib.h, which causes
functions like strtoll_l() or sscanf_l() to not be declared.

When compiling with -fmodules, locale_base_api.h is processed separately
due to a declaration in /usr/include/c++/v1/module.modulemap, and this
will cause errors due to the above undeclared symbols.

Meanwhile, upstream has substantially reorganized this part of libc++'s
headers, so apply a minimalistic workaround: specifically when compiling
with -fmodules, add includes of stdio.h and stdlib.h.

PR: 286342
MFC after: 1 week