cmd/internal/obj/x86: fix nacl/amd64p32 build

some x86 instructions (e.g. PINSRW) might store memory address in Prog.From3,
so we must also rewrite Prog.From3 on nacl.

Change-Id: I2a0da0f692ba321eba17fbc454d68aaafa133515
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7074
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/internal/obj/x86/obj6.go

@@ -191,6 +191,7 @@ func progedit(ctxt *obj.Link, p *obj.Prog) {
 	}
 
 	if ctxt.Headtype == obj.Hnacl && p.Mode == 64 {
+		nacladdr(ctxt, p, &p.From3)
 		nacladdr(ctxt, p, &p.From)
 		nacladdr(ctxt, p, &p.To)
 	}