go.test: update issue4085b.go to version from Go 1.13

This lets the test pass on AIX.

Not updating to master because we get slightly different error messages,
so that is a more complex change.

gcc/testsuite/go.test/test/fixedbugs/issue4085b.go

@@ -21,9 +21,11 @@ func main() {
 	shouldPanic("cap out of range", func() { _ = make(T, 0, int64(n)) })
 	var t *byte
 	if unsafe.Sizeof(t) == 8 {
-		var n2 int64 = 1 << 50
+		// Test mem > maxAlloc
+		var n2 int64 = 1 << 59
 		shouldPanic("len out of range", func() { _ = make(T, int(n2)) })
 		shouldPanic("cap out of range", func() { _ = make(T, 0, int(n2)) })
+		// Test elem.size*cap overflow
 		n2 = 1<<63 - 1
 		shouldPanic("len out of range", func() { _ = make(T, int(n2)) })
 		shouldPanic("cap out of range", func() { _ = make(T, 0, int(n2)) })