cmd/compile: record regabi status in DW_AT_producer

Records if regabi was enabled during compilation in the DW_AT_producer
attribute of each compile unit.
This is useful to debuggers that support the debugCall protocol.

Change-Id: I5ad2c48ebf126aeb8bfb459b53a1a5304550036a
Reviewed-on: https://go-review.googlesource.com/c/go/+/318050


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>