Add stricter checks on user changes to SSTATUS.

Rather than trying to blacklist which bits userland can't write to via
sigreturn() or setcontext(), only permit changes to whitelisted bits.

• Permit arbitrary writes to bits in the user-writable USTATUS register that shadows SSTATUS.

• Ignore changes in write-only bits maintained by the CPU.

• Ignore the user-supplied value of the FS field used to track floating point state and instead set it to a value matching the actions taken by set_fpcontext().

Discussed with: mhorne
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23338