Disable write protection around patching of XSAVE instruction in the
context switch code.

Some BIOSes give control to the OS with CR0.WP already set, making the
kernel text read-only before cpu_startup().

Reported by: Peter Lei <peter.lei@ieee.org>
Reviewed by: jtl
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D14768