runtime: unify fetching of locals and arguments maps

Currently we have two nearly identical copies of the code that fetches
the locals and arguments liveness maps for a frame, plus a third
that's a poor knock-off. Unify these all into a single function.

Change-Id: Ibce7926a0b0e3d23182112da4e25df899579a585
Reviewed-on: https://go-review.googlesource.com/109698


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>