amd64: Fix output operand specs for the stmxcsr and vmread intrinsics

This does not appear to affect code generation, at least with the
default toolchain.

Noticed because incorrect output specifications lead to false positives
from KMSAN, as the instrumentation uses them to update shadow state for
output operands.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e54ae8258d6433ad2f2411dbeaa1fde6b817d5ef)