cmd/go, cmd/cgo: pass "-mabi=64" to gcc on mips64

Change-Id: I9ac2ae57a00cee23d6255db02419b0a0f087d4f3
Reviewed-on: https://go-review.googlesource.com/22801Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>

src/cmd/cgo/gcc.go

@@ -1089,6 +1089,8 @@ func (p *Package) gccMachine() []string {
 		return []string{"-m31"}
 	case "s390x":
 		return []string{"-m64"}
+	case "mips64", "mips64le":
+		return []string{"-mabi=64"}
 	}
 	return nil
 }

src/cmd/go/build.go

@@ -3103,6 +3103,8 @@ func (b *builder) gccArchArgs() []string {
 		return []string{"-marm"} // not thumb
 	case "s390x":
 		return []string{"-m64", "-march=z196"}
+	case "mips64", "mips64le":
+		return []string{"-mabi=64"}
 	}
 	return nil
 }