We already turn the AMD erratum383 workaround on for certain VM_GUEST_VM
if specific CPU features are not present.
Some simulation environments, e.g. gem5, have been found to require more
TLB management from the kernel in certain setups. It is currently unclear why.
Turning on the workaround_erratum383 seems to help and make problems (panics)
go away.
Given this is a fairly uncommon environment so far, allowing the workaround
to be manually enabled from loader in order to make debugging and comparing
traces easier, but also to allow gem5 run FreeBSD in X86 timing mode, seems
to be the least intrusive option for now until the issue if fully understood.

Sponsored by: DARPA/AFRL
Reviewed by: kib, alc (earlier)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6206