asan: Use memset_early() to fill shadow memory

builtin_memset() calls are replaced with calls to memset(), but that
can't be used before ifunc relocations are processed if the
implementation is selected at boot time. Meanwhile, the sanitizer may
emit calls to asan_set_shadow_*() as soon as locore jumps into C code,
before ifuncs are selected.

Just unconditionally use memset_early() to work around this.

Reported by: andrew
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55079