libthr: Add src.conf variable WITHOUT_PTHREADS_ASSERTIONS

This patch fixes a bug which prevents building libthr without
_PTHREADS_INVARIANTS defined. The default remains to build libthr
with -D_PTHREADS_INVARIANTS. However, with this patch, if one builds
libthr with WITHOUT_PTHREADS_ASSERTIONS=true then the latency to
acquire+release a default pthread mutex is reduced by roughly 5%, and a
robust mutex by roughly 18% (as measured by a simple synthetic test on a
Xeon E5-2697a based machine).

Reviewed by: jhb, kib, mjg
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D40900