[dev.simd] simd: add some emulations to amd64

floating point abs
int8xNN Mul
uint8xNN Mul

Change-Id: I2e74dd6cbe6dae800c7cf1f5e9456ba7b5b8e7cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/781261
TryBot-Bypass: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>