cmd/link: handle R_GOTPCREL separately on darwin

To generate the correct section offset the shared code path for
R_CALL, R_PCREL, and R_GOTPCREL on darwin when externally linking
walks up the symbol heirarchy adding the differences. This is fine,
except in the case where we are generating a GOT lookup, because
the topmost symbol is left in r.Xsym instead of the symbol we are
looking up. So all funcsym GOT lookups were looking up the outer
"go.func.*" symbol.

Fix this by separating out the R_GOTPCREL code path.

For #17828 (and may fix it).

Change-Id: I2c9f4d135e77c17270aa064d8c876dc6d485d659
Reviewed-on: https://go-review.googlesource.com/33211
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testplugin/src/plugin1/plugin1.go

@@ -17,9 +17,17 @@ func ReadCommonX() int {
 
 var Seven int
 
+func call(fn func()) {
+	fn()
+}
+
+func g() {
+	common.X *= Seven
+}
+
 func init() {
 	Seven = 7
-	common.X *= Seven
+	call(g)
 }
 
 func main() {

src/cmd/link/internal/ld/data.go

@@ -599,7 +599,19 @@ func relocsym(ctxt *Link, s *Symbol) {
 			}
 
 			// r->sym can be null when CALL $(constant) is transformed from absolute PC to relative PC call.
-		case obj.R_CALL, obj.R_GOTPCREL, obj.R_PCREL:
+		case obj.R_GOTPCREL:
+			if ctxt.DynlinkingGo() && Headtype == obj.Hdarwin && r.Sym != nil && r.Sym.Type != obj.SCONST {
+				r.Done = 0
+				r.Xadd = r.Add
+				r.Xadd -= int64(r.Siz) // relative to address after the relocated chunk
+				r.Xsym = r.Sym
+
+				o = r.Xadd
+				o += int64(r.Siz)
+				break
+			}
+			fallthrough
+		case obj.R_CALL, obj.R_PCREL:
 			if Linkmode == LinkExternal && r.Sym != nil && r.Sym.Type != obj.SCONST && (r.Sym.Sect != s.Sect || r.Type == obj.R_GOTPCREL) {
 				r.Done = 0