[dev.link] cmd/link: clean up some pclntab state

[dev.link] cmd/link: clean up some pclntab state

Clean up some pclntab state, specifically:
1) Remove the oldPclnState type.
2) Move a structure out of pclnState, that was holding some memory.
3) Stop passing container around everywhere and calling emitPcln. Use a
   slice of function symbols instead.

Change-Id: I74e916564cd769a706750d024e55ee0d811a79da
Reviewed-on: https://go-review.googlesource.com/c/go/+/248379
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>