internal/simdgen: compute CPU feature in XED decoder

Currently, the XED decoder emits the raw "EXTENSION" and "ISA_SET"
fields directly from the XED, and these are translated into a CPU
feature by godefs using a bunch of fairly ad hoc string manipulations.

Replace this with computing the CPU feature directly in the XED
decoder. The extension and isa_set are strictly XED concepts, while
"CPU features" are generic concepts. Thus, this should be the role of
the XED decoder.

We also use an explicit mapping table rather than string
manipulations. These CPU feature names appear in the API, and thus
it's important that we pay attention to their names.

No effect on generated code.

Change-Id: I1c7c79c461d57b2cd78cfa81f376683ae33c69b1
Reviewed-on: https://go-review.googlesource.com/c/arch/+/694858
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Austin Clements <austin@google.com>