These are required by ISO/IEC 9899:2024 § 7.18.1 ¶ 1 but were forgotten
in my initial work.
The current approach leaks intptr_t, uintptr_t, intmax_t, and uintmax_t
through <sys/_stdint.h>. This could be avoided using a more complicated
approach if desired.
PR: 294131
Reported by: Collin Funk <collin.funk1@gmail.com>
MFC after: 1 week