liblink: shorter encoding for zeroing register

Encode MOV $0, %ax as XOR %eax, %eax instead of
XOR %rax, %rax. If an operand register does not
need REX.w bit (i.e. not one of R8-R15), it is
encoded in 2 bytes instead of 3 bytes.

LGTM=rsc
R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/115580044

src/liblink/asm6.c

@@ -3066,6 +3066,7 @@ found:
 		break;
 
 	case Zclr:
+		ctxt->rexflag &= ~Pw;
 		*ctxt->andptr++ = op;
 		asmand(ctxt, &p->to, &p->to);
 		break;