[dev.link] cmd/link: parallelize ELF relocation writing

[dev.link] cmd/link: parallelize ELF relocation writing

Now that we write ELF relocation records in mapped memory with
known sizes and offsets, we can write them in parallel.

Further speed up Asmb2 pass. Linking cmd/compile with external
linking,

Asmb2        141ms ± 4%      97ms ± 5%  -30.98%  (p=0.000 n=10+9)

Change-Id: I52c2b9230e90ed4421c21d7ef13a4f1e996f6054
Reviewed-on: https://go-review.googlesource.com/c/go/+/240400
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>