Implement math/bits.TrailingZerosX using intrinsics.

Generally reorganize the intrinsic spec a bit.
The instrinsics data structure is now built at init time.
This will make doing the other functions in math/bits easier.

Update sys.CtzX to return int instead of uint{64,32} so it
matches math/bits.TrailingZerosX.

Improve the intrinsics a bit for amd64.  We don't need the CMOV
for <64 bit versions.

Update #18616

Change-Id: Ic1c5339c943f961d830ae56f12674d7b29d4ff39
Reviewed-on: https://go-review.googlesource.com/38155
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>