cmd/go: disable fuzz instrumentation for internal/godebug

This is thought to be the cause of certain recent longtest failures.
Let's try it out.

This appears to fix the longtests fuzz failures. I suspect that the
sync.Map in internal/godebug is at fault with the implementation
changing. I'm not sure yet exactly why this is a problem, maybe inlining
that didn't happen before? I don't know exactly when coverage
instrumentation happens in the compiler, but this is definitely the
problem.

For good measure, let's add internal/sync. If sync is on the list,
internal/sync should be, too.

Fixes #70429.
Fixes #70430.
Fixes #70431.

Change-Id: Ic9f49daa0956e3a50192bcc7778983682b5d12b8
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/629475
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/test/test.go

@@ -933,7 +933,9 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 		var skipInstrumentation = map[string]bool{
 			"context":               true,
 			"internal/fuzz":         true,
+			"internal/godebug":      true,
 			"internal/runtime/maps": true,
+			"internal/sync":         true,
 			"reflect":               true,
 			"runtime":               true,
 			"sync":                  true,