internal/runtime/maps: speed up Clear

internal/runtime/maps: speed up Clear

We don't need to know the actual full slots, just whether there
are any or not.

The test for any full slots is simpler on amd64. We don't have to
use PMOVMSKB and do the intreg->floatreg transfer.

Fixes #75097

Change-Id: Iace1c100618d7fc2ac5ddd5fe9e8fe5c9595243f
Reviewed-on: https://go-review.googlesource.com/c/go/+/697875
Reviewed-by: Youlin Feng <fengyoulin@live.com>
Auto-Submit: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>