elf: Avoid dumping uninitialized bytes in PRSTATUS core dump notes

elf_prstatus_t contains pad space.

Reported by: KMSAN
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34606

(cherry picked from commit 1babcad6bc4093664a10c7ab98b9ebd2d64bcaaf)