[dev.link] cmd/link: parallelize reloc sym

[dev.link] cmd/link: parallelize reloc sym

I tried a couple of different architectures (goroutine per symbol, 8
goroutines handling symbols from channels, and this architecture), and
this was the best. Another possible approach could be to divide up the
space of relocations, forgo the channels, and just pass slices to the
relocation routines, which would possibly be faster.

Reloc                     13.9ms ± 5%      9.0ms ±10%       -35.09%  (p=0.000 n=8+9)

Change-Id: I5111220e855313fae4b89d64277759c4dc33b697
Reviewed-on: https://go-review.googlesource.com/c/go/+/220842
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>