[dev.link] cmd/link, cmd/compile: create content addressable pcdata syms

[dev.link] cmd/link, cmd/compile: create content addressable pcdata syms

Switch pcdata over to content addressable symbols. This is the last
step before removing these from pclntab_old.

No meaningful benchmarks changes come from this work.

Change-Id: I3f74f3d6026a278babe437c8010e22992c92bd89
Reviewed-on: https://go-review.googlesource.com/c/go/+/247399
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>