cmd/go: add math/bits to runtime packages in TestNewReleaseRebuildsStalePackagesInGOPATH

This fixes a test failure introduced in CL 190620.

Updates #35112

Change-Id: I568ae85a456ccd8103563b0ce2e42b7348776a5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/205877
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/cmd/go/go_test.go

@@ -917,6 +917,7 @@ func TestNewReleaseRebuildsStalePackagesInGOPATH(t *testing.T) {
 		"src/runtime",
 		"src/internal/bytealg",
 		"src/internal/cpu",
+		"src/math/bits",
 		"src/unsafe",
 		filepath.Join("pkg", runtime.GOOS+"_"+runtime.GOARCH),
 		filepath.Join("pkg/tool", runtime.GOOS+"_"+runtime.GOARCH),