Use sigfastblock(2) for masking signals in libthr.

Ensure proper handshake to transfer sigfastblock(2) blocking word
ownership from rtld to libthr.

Unfortunately sigfastblock(2) is not enough to stop intercepting
signals in libthr, because critical sections must ensure more than
just signal blocking.

Tested by: pho
Disscussed with: cem, emaste, jilles
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D12773