cmd/compile: optimize 386's math.bits.TrailingZeros16

cmd/compile: optimize 386's math.bits.TrailingZeros16

This CL optimizes math.bits.TrailingZeros16 on 386 with
a pair of BSFL and ORL instrcutions.

The case TrailingZeros16-4 of the benchmark test in
math/bits shows big improvement.
name               old time/op  new time/op  delta
TrailingZeros16-4  1.55ns ± 1%  0.87ns ± 1%  -43.87%  (p=0.000 n=50+49)

Change-Id: Ia899975b0e46f45dcd20223b713ed632bc32740b
Reviewed-on: https://go-review.googlesource.com/c/go/+/189277
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>