build-infra: --disable-precompiled-headers does not seem to work

Even if you set --disable-precompiled-headers, we seem to generate precompiled headers. At least the log output indicated this.

Try a search for "Generating precompiled header" in vm.make in hotspot and see how that interacts with our flag.

If we can't get this to work, we should remove the flag. Having a flag that does not do what it advertises is really really bad.