Use the recently added CPU feature detection API rather than custom
assembly. This will need to be updated to use 'internal/cpu' when
the package is revendored into std.

Change-Id: Icf0f28236ca0c351c88035f36e5b9c29d4bc2ba7
Reviewed-on: https://go-review.googlesource.com/c/164383Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>