amd64: for small cores, use (big hammer) INVPCID_CTXGLOB instead of INVLPG

A hypothetical CPU bug makes invalidation of global PTEs using INVLPG
in pcid mode unreliable, it seems. The workaround is applied for all
CPUs with small cores, since we do not know the scope of the issue, and
the right fix.

Reviewed by: alc (previous version)
Discussed with: emaste, markj
Tested by: karels
PR: 261169, 266145
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37770