Use the recently added CPU feature detection API rather than custom
assembly.

While we are here also make vet happy by renaming params in the asm
file to chain to match the go function declaration.

Change-Id: Ide0171793c9fa5ef6671b394e97a27f2c6e44a9f
Reviewed-on: https://go-review.googlesource.com/c/164381Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>