runtime: improve scan inner loop

runtime: improve scan inner loop

On every arch except amd64, it is faster to do x&(x-1) than x^(1<<n).

Most archs need 3 instructions for the latter: MOV $1, R; SLL n, R;
ANDN R, x. Maybe 4 if there's no ANDN.

Most archs need only 2 instructions to do x&(x-1). It takes 3 on
x86/amd64 because NEG only works in place.

Only amd64 can do x^(1<<n) in a single instruction.
(We could on 386 also, but that's currently not implemented.)

Change-Id: I3b74b7a466ab972b20a25dbb21b572baf95c3467
Reviewed-on: https://go-review.googlesource.com/c/go/+/672956
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>