Add known instruction set variants to logging strings

If a user had a wrong variant, they would get a message like
`Unexpected CPU variant for X86 using defaults: x86` but they
wouldn't know which ones we accept, unless they take a look at
the code.

This change comes after a developer's suggestion when encountering
said warning string.

Test: Presubmit tests
Change-Id: I3a7b9408cf55114229843f6ff6013999350d7185
4 files changed