cmd/5g, cmd/5l, cmd/ld: Small adjustments for the Plan 9 native tools

A few USED(xxx) additions and a couple of deletions of variable
initialisations that go unused.  One questionable correction,
mirrored in 8l/asm.c, where the result of invocation of a function
shouldn't be used.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6736054

src/cmd/5g/ggen.c

@@ -543,6 +543,7 @@ cgen_shift(int op, int bounded, Node *nl, Node *nr, Node *res)
 	Type *tr;
 	uvlong sc;
 
+	USED(bounded);
 	if(nl->type->width > 4)
 		fatal("cgen_shift %T", nl->type);
 

src/cmd/5g/reg.c

@@ -207,7 +207,6 @@ regopt(Prog *firstp)
 		return;
 	}
 
-	r1 = R;
 	firstr = R;
 	lastr = R;
 

src/cmd/5l/asm.c

@@ -913,6 +913,7 @@ asmb(void)
 			}
 			phsh(pnote, sh);
 		}
+		USED(resoff);
 
 		elfphload(&segtext);
 		elfphload(&segdata);

src/cmd/ld/pe.c

@@ -536,6 +536,7 @@ addexcept(IMAGE_SECTION_HEADER *text)
 	uvlong n;
 	Sym *sym;
 
+	USED(text);
 	if(thechar != '6')
 		return;