[dev.link] cmd/link: parallelize asmb on amd64

Introduces a parallel OutBuf implementation, and POC on amd64. Due to
some of the weird behaviors I saw on MacOS (SIGBUS while calling msync),
I will wait for feedback to port to other architectures.

On my mac, sped up Asmb by ~78% for cmd/compile (below). Will likely
have an appreciable speedup on kubelet benchmark.

Asmb                      39.1ms ±11%     8.5ms ±10%     -78.17%  (p=0.000 n=10+9)
TotalTime                  596ms ± 2%     577ms ± 8%      -3.07%  (p=0.034 n=8+10)

Change-Id: Id2a2577c3f4da155d8dccc862897f43b941877ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/223742


Run-TryBot: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>