cmd/link: run generator functions along with symbol writing

Like we do for applying relocations, for generator symbols, run
the generator function along with symbol writing. This will
probably have better locality and parallelism.

Linking cmd/compile,

Asmb          29.9ms ± 5%    19.1ms ±12%  -36.18%  (p=0.000 n=10+9)
TotalTime      351ms ± 3%     339ms ± 2%   -3.51%  (p=0.000 n=11+10)

Change-Id: I9cda6718bf70b3bcf1b7a501a845d6136234d2ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/263640


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