internal/abi: use arch family instead of arch string

internal/abi: use arch family instead of arch string

No point in using string comparison when we can use integer comparison instead.

Unify the constants in cmd/internal/sys and internal/goarch while
we are at it.

Change-Id: I5681a601030307b7b286f958a8965559cb43506d
Reviewed-on: https://go-review.googlesource.com/c/go/+/652175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Keith Randall <khr@google.com>