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/1752Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/9l/9.out.h

@@ -106,9 +106,9 @@ enum
 	C_ANDCON,	/* 0 < v <= 0xFFFF */
 	C_LCON,		/* other 32 */
 	C_DCON,		/* other 64 (could subdivide further) */
-	C_SACON,
+	C_SACON,	/* $n(REG) where n is small */
 	C_SECON,
-	C_LACON,
+	C_LACON,	/* $n(REG) where n is large */
 	C_LECON,
 	C_SBRA,
 	C_LBRA,

src/liblink/obj9.c

@@ -145,7 +145,8 @@ progedit(Link *ctxt, Prog *p)
 		}
 		break;
 	case AMOVD:
-		if(p->from.type == D_CONST && p->from.name == D_NONE && (int64)(uint32)p->from.offset != p->from.offset) {
+		// Put >32-bit constants in memory and load them
+		if(p->from.type == D_CONST && p->from.name == D_NONE && p->from.reg == NREG && (int32)p->from.offset != p->from.offset) {
 			sprint(literal, "$i64.%016llux", (uvlong)p->from.offset);
 			s = linklookup(ctxt, literal, 0);
 			s->size = 8;