vmm(4): Mask Spectre feature bits on AMD hosts

For parity with Intel hosts, which already mask out the CPUID feature
bits that indicate the presence of the SPEC_CTRL MSR, do the same on
AMD.

Eventually we may want to have a better support story for guests, but
for now, limit the damage of incorrectly indicating an MSR we do not yet
support.

Eventually, we may want a generic CPUID override system for
administrators, or for minimum supported feature set in heterogenous
environments with failover. That is a much larger scope effort than
this bug fix.

PR: 235010
Reported by: Rys Sommefeldt <rys AT sommefeldt.com>
Sponsored by: Dell EMC Isilon