amd64: preserve %cr2 in NMI/MCE/DBG handlers.

These handlers could interrupt code which has interrupts disabled,
and if a spurious page fault occurs during exception handler run,
we get clobbered %cr2 in higher level stack.

This is mostly a speculation, but it is based on hints from good sources.

MFC after: 1 week
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27772