cmd/internal/obj: use string instead of LSym in Pcln

cmd/internal/obj: use string instead of LSym in Pcln

In a concurrent backend, Ctxt.Lookup will need some
form of concurrency protection, which will make it
more expensive.

This CL changes the pcln table builder to track
filenames as strings rather than LSyms.
Those strings are then converted into LSyms
at the last moment, for writing the object file.

This CL removes over 85% of the calls to Ctxt.Lookup
in a run of make.bash.

Passes toolstash-check.

Updates #15756

Change-Id: I3c53deff6f16f2643169f3bdfcc7aca2ca58b0a4
Reviewed-on: https://go-review.googlesource.com/39291
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>