amd64: MCOUNT_EXIT: restore interrupts, don't unconditionally reenable them

amd64: MCOUNT_EXIT: restore interrupts, don't unconditionally reenable them

This bug can cause all sorts of problems, but in particular it was
most easily reproduced as a double fault in the syscall return path on
this CPU model:

Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz

Tons of help from guenther@ in narrowing down the root cause.  Fix
tweaked by guenther@.  Additional input from deraadt@ and kettenis@.

ok guenther@