MFC 238311:

MFC 238311:
Add a clts() wrapper around the 'clts' instruction to <machine/cpufunc.h>
on x86 and use that to implement stop_emulating() in the fpu/npx code.
Reimplement start_emulating() in the non-XEN case by using load_cr0() and
rcr0() instead of the 'lmsw' and 'smsw' instructions.  Intel explicitly
discourages the use of 'lmsw' and 'smsw' on 80386 and later processors in
the description of these instructions in Volume 2 of the ADM.

git-svn-id: svn://svn.freebsd.org/base/stable/9@239914 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f