[dev.link] cmd/link: use generator symbols for the rest of pclntab

[dev.link] cmd/link: use generator symbols for the rest of pclntab

Move the rest of pclntab creation to generator symbols. Any savings in
pclntab generation CPU time is eaten by the generators run in Asmb
phase.

Stats for Darwin, cmd/compile:

alloc/op:
Pclntab_GC                   13.9MB ± 0%     6.4MB ± 0%    -53.68%  (p=0.000 n=10+10)

allocs/op
Pclntab_GC                    86.5k ± 0%     61.5k ± 0%    -28.90%  (p=0.000 n=10+10)

liveB:
Pclntab_GC                    24.3M ± 0%     22.9M ± 0%     -5.57%  (p=0.000 n=10+10)

Timing:

Pclntab                   32.1ms ± 2%    24.2ms ± 2%    -24.35%  (p=0.000 n=9+9)
Asmb                      18.3ms ±14%    27.4ms ± 9%    +49.55%  (p=0.000 n=10+10)
TotalTime                  351ms ± 2%     347ms ± 3%       ~     (p=0.200 n=9+8)

Change-Id: I5c6b6df5953f6f255240e07578f1c9f8c5f68500
Reviewed-on: https://go-review.googlesource.com/c/go/+/249023
Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>