libc/aarch64: fix ELF size of timingsafe_memcmp

Looks like a copy and paste error.
The ELF size of 0 prevents Valgrind from redirecting this function.

PR: 289845
Reviewed by: fuz
Fixes: 3f224333af163d5fcd7547a20993dcf18f19076c
See also: https://bugs.kde.org/show_bug.cgi?id=509406
Pull Request: https://github.com/freebsd/freebsd-src/pull/1854
MFC after: 1 day
Event: EuroBSDcon 2025