cmd/compile: use TZCNT instruction for GOAMD64>=v3

cmd/compile: use TZCNT instruction for GOAMD64>=v3

on my Intel CoffeeLake CPU:
name               old time/op  new time/op  delta
TrailingZeros-8    0.68ns ± 1%  0.64ns ± 1%  -6.26%  (p=0.000 n=10+10)
TrailingZeros8-8   0.70ns ± 1%  0.70ns ± 1%    ~     (p=0.697 n=10+10)
TrailingZeros16-8  0.70ns ± 1%  0.70ns ± 1%  +0.57%  (p=0.043 n=10+10)
TrailingZeros32-8  0.66ns ± 1%  0.64ns ± 1%  -3.35%  (p=0.000 n=10+10)
TrailingZeros64-8  0.68ns ± 1%  0.64ns ± 1%  -5.84%  (p=0.000 n=9+10)

Updates #45453

Change-Id: I228ff2d51df24b1306136f061432f8a12bb1d6fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/353249
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>