cmd/ld: fix another unsigned value causing bugs on Plan 9

"The usual conversions" bite again.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/9103044

src/cmd/ld/data.c

@@ -247,7 +247,13 @@ relocsym(Sym *s)
 			o = 0;
 			if(r->sym)
 				o += symaddr(r->sym);
-			o += r->add - (s->value + r->off + r->siz);
+			// NOTE: The (int32) cast on the next line works around a bug in Plan 9's 8c
+			// compiler. The expression s->value + r->off + r->siz is int32 + int32 +
+			// uchar, and Plan 9 8c incorrectly treats the expression as type uint32
+			// instead of int32, causing incorrect values when sign extended for adding
+			// to o. The bug only occurs on Plan 9, because this C program is compiled by
+			// the standard host compiler (gcc on most other systems).
+			o += r->add - (s->value + r->off + (int32)r->siz);
 			break;
 		case D_SIZE:
 			o = r->sym->size + r->add;