cmd/internal/obj: consolidate emitting entry stack map

The obj package needs to emit the PCDATA to select the entry stack map
before calling morestack. Currently this is copied for every
architecture. Since we're about to change how this works, consolidate
all of these copies into a single helper function.

For #24543.

Change-Id: Ia92d94de78f8e23fd06dba747c43e03e5989f67b
Reviewed-on: https://go-review.googlesource.com/109346


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