[libc++] Only include <setjmp.h> from the C library if it exists (#81887)

In 2cea1bab, we removed the <setjmp.h> header provided by libc++. However, we did not conditionally include the underlying <setjmp.h>
header only if the C library provides one, which we otherwise do consistently (see e.g. 647ddc08).

rdar://122978778