Do not call FreeBSD-ABI specific code for all ABIs

Use sysentvec hooks to only call umtx_thread_exit/umtx_exec, which handle
robust mutexes, for native FreeBSD ABI. Similarly, there is no sense
in calling sigfastblock_clear() for non-native ABIs.

Requested by: dchagin
Reviewed by: dchagin, markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D30987