cmd/internal/obj: refactor ppc64 address relocation code generation

The code for generating a long constant versus generating an address
(either via a relocation, or known offset) should be handled in the
same place.

Resolve this by classifying memory arguments as C_LACON (a long
address constant) instead of C_LCON (a long constant).

Likewise, reorder AMOVD/AMOVW optab entries to keep similar
classifications near each other. An extra optab entry for
DWORD is also added to continue handling C_LACON arguments
correctly.

Change-Id: I5ce28400492a071f615125a9b8d260826f1600d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/312296


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>
Trust: Carlos Eduardo Seo <carlos.seo@linaro.org>