cmd/compile/internal/ssagen: improve intrinsic architecture handling

The architecture handling code for intrinsics is more complex than
it needs to be. sys.Archs is already an array of *sys.Arch and the
existing InFamily function can be used instead of a reimplementation.

Add some test coverage for sys.Arch.InFamily while here.

Change-Id: Ia764f211114fea65424c09a421c5ccb02b7187b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/605476


Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>