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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31466