cmd/link: make PCIter available to compiler

I'm branching this off cl/187117, and will be reworking that diff stack.

Testing: I've run go build -toolexec 'toolstash -cmp'

Change-Id: I922a97d0f25d52ea70cd974008a063d4e7af34a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/188023


Reviewed-by: Austin Clements <austin@google.com>