test: fix the wrong test of math.Copysign(c, -1) for arm64

The CL 132915 added the wrong codegen test for math.Copysign(c, -1),
it should test that AND is not emitted. This CL fixes this error.

Change-Id: Ida1d3d54ebfc7f238abccbc1f70f914e1b5bfd91
Reviewed-on: https://go-review.googlesource.com/134815Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/codegen/math.go

@@ -74,7 +74,7 @@ func copysign(a, b, c float64) {
 	// amd64:"BTSQ\t[$]63"
 	// s390x:"LNDFR\t",-"MOVD\t"     (no integer load/store)
 	// ppc64le:"FCPSGN"
-	// arm64:"ORR\t[$]-9223372036854775808"
+	// arm64:"ORR", -"AND"
 	sink64[1] = math.Copysign(c, -1)
 
 	// Like math.Copysign(c, -1), but with integer operations. Useful