libc: implement rtld_get_stack_prot() for real

which makes stack prot correct for non-main threads created by binaries
with statically linked libthr.

Cache result, but do not engage into the full double-checked locking,
since calculation of the return value is idempotent.

PR: 252549
Reported and reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28075