cmd/internal/obj/x86: On amd64, relocation type for and indirect call is pc-relative.

With this change, the code in bug #15609 compiles and runs properly:

0000000000401070 <main.jump>:
  401070:	ff 15 aa 7e 06 00    	callq  *0x67eaa(%rip)        # 468f20 <main.pointer>
  401076:	c3                   	retq

0000000000468f20 g     O .rodata	0000000000000008 main.pointer

Fixes #15609

Change-Id: Iebb4d5a9f9fff335b693f4efcc97882fe04eefd7
Reviewed-on: https://go-review.googlesource.com/22950
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/obj/x86/asm6.go

@@ -3691,7 +3691,11 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 				ctxt.AsmBuf.Put2(byte(op), o.op[z+1])
 				r = obj.Addrel(ctxt.Cursym)
 				r.Off = int32(p.Pc + int64(ctxt.AsmBuf.Len()))
-				r.Type = obj.R_ADDR
+				if p.Mode == 64 {
+					r.Type = obj.R_PCREL
+				} else {
+					r.Type = obj.R_ADDR
+				}
 				r.Siz = 4
 				r.Add = p.To.Offset
 				r.Sym = p.To.Sym

test/fixedbugs/issue15609.dir/call.go

+// +build !amd64,!386
+
+package main
+
+func jump() {
+	target()
+}

test/fixedbugs/issue15609.dir/call_386.s

+#include "textflag.h"
+
+DATA ·pointer(SB)/4, $·target(SB)
+GLOBL ·pointer(SB),RODATA,$4
+
+TEXT ·jump(SB),NOSPLIT,$4
+        CALL *·pointer(SB)
+        RET

test/fixedbugs/issue15609.dir/call_amd64.s

+#include "textflag.h"
+
+DATA ·pointer(SB)/8, $·target(SB)
+GLOBL ·pointer(SB),RODATA,$8
+
+TEXT ·jump(SB),NOSPLIT,$8
+        CALL *·pointer(SB)
+        RET

test/fixedbugs/issue15609.dir/call_decl.go

+// +build amd64 386
+
+package main
+
+func jump()

test/fixedbugs/issue15609.dir/main.go

+package main
+
+var called bool
+
+func target() {
+	called = true
+}
+
+func main() {
+	jump()
+	if !called {
+		panic("target not called")
+	}
+}