cmd/compile: optimize Ctz64 on 386

cmd/compile: optimize Ctz64 on 386

Compared with the version generated by dec64.rules based on Ctz32,
the number of assembly instructions is reduced by half.

SwissMap uses TrailingZeros64 to find the first match in its control
group and may benefit from this CL on 386 architectures.

goos: linux
goarch: 386
cpu: 13th Gen Intel(R) Core(TM) i7-13700H
                   │   old.txt    │               new.txt                │
                   │    sec/op    │    sec/op     vs base                │
TrailingZeros64-20   0.8828n ± 1%   0.6299n ± 1%  -28.65% (p=0.000 n=20)

Change-Id: Iba08a3f4e13efd3349715dfb7fcd5fd470286cd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/624376
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>