[dev.simd] internal/runtime/gc: add simd package based greentea kernels

[dev.simd] internal/runtime/gc: add simd package based greentea kernels

This CL adds a new generator to internal/runtime/gc/scan that generates expander
kernels in Go SIMD. This CL also includes a Go SIMD scan kernel and a
Go SIMD filter kernel.

This CL also includes the plumbing, it will use the Go SIMD kernels if
goexperiment.simd is on.

Benchmark results:
...
ScanSpanPacked/cache=tiny/pages=1/sizeclass=26/pct=80-88     354.8n ±  1%   272.4n ±  0%  -23.22% (p=0.002 n=6)
ScanSpanPacked/cache=tiny/pages=1/sizeclass=26/pct=90-88     375.7n ±  0%   287.1n ±  0%  -23.58% (p=0.002 n=6)
ScanSpanPacked/cache=tiny/pages=1/sizeclass=26/pct=100-88    450.0n ±  1%   327.4n ±  0%  -27.24% (p=0.002 n=6)
geomean                                                      246.5n         199.4n        -19.10%

Throughput +25%.

Change-Id: Ib85e01b7de18181db9e7b6026863209a993aa85f
Reviewed-on: https://go-review.googlesource.com/c/go/+/719520
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>