x86: handle MXCSR from XSAVEOPT when x87 state was optimized
PR: 275322
Reported by: Cheyenne Wills <cheyenne.wills@gmail.com>
Reviewed by: emaste, jhb, olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44522