Consistently ensure that we do not load MXCSR with reserved bits set.

Some callers of fpusetregs()/npxsetregs(), most importantly
set_fpcontext(), clear reserved bits. But some did not. Do the
clearing in fpusetregs() and remove now redundand operation from
set_fpcontext().

Reported by: Maxime Villard <max@m00nbsd.net>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week