cmd/compile: add DWARF reg defs & fix 32-bit location list bug

Before DWARF location lists can be turned on, 3 bugs need
fixing.

This CL addresses two -- lack of register definitions for
various architectures, and bugs on 32-bit platforms.
The third bug comes later.

Passes
GO_GCFLAGS=-dwarflocationlists ./run.bash -no-rebuild
(-no-rebuild because the map dependence causes trouble)

Change-Id: I4223b48ade84763e4b048e4aeb81149f082c7bc7
Reviewed-on: https://go-review.googlesource.com/99255


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>