Define stdint.h macros unconditionally

Similar to glibc, define all the stdint.h macros such as SIZE_MAX,
UINT64_C, etc unconditionally. I.e. no longer check whether
__STDC_CONSTANT_MACROS or __STDC_LIMIT_MACROS are defined. See also
https://sourceware.org/bugzilla/show_bug.cgi?id=15366.

This is part of reverting base 00bee6fcd77f, which reverted an upstream
libc++ commit that eliminated libc++'s stdint.h wrapper header.

Submitted by: Nikolas Klauser <nikolasklauser@berlin.de>
MFC after: 1 week
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D56746

(cherry picked from commit 966fb94cb3575ccd39da211165d0858fd0eb0ef2)