liblink: generate correct code for MOVD $-n(Rm), x on ppc64

On ppc64, liblink rewrites MOVD's of >32-bit constants by putting the
constant in memory and rewriting the MOVD to load from that memory
address.  However, there were two bugs in the condition:

a) owing to an incorrect sign extension, it triggered for all negative
   constants, and

b) it could trigger for constant offsets from registers (addresses of
   the form $n(Rm) in assembly)

Together, these meant instructions of the form MOVD $-n(Rm), x were
compiled by putting -n in memory and rewriting the MOVD to load this
constant from memory (completely dropping Rm).

Change-Id: I1f6cc980efa3e3d6f164b46c985b2c3b55971cca
Reviewed-on: https://go-review.googlesource.com/1752
Reviewed-by: Minux Ma <minux@golang.org>