[libc++] Simplify the implementation of <stddef.h> (#86843)

Libc++'s own <stddef.h> is complicated by the need to handle various
platform-specific macros and to support duplicate inclusion. In reality,
we only need to add a declaration of nullptr_t to it, so we can simply
include the underlying <stddef.h> outside of our guards to let it handle
re-inclusion itself.

(cherry picked from commit 2950283d)