runtime: make test for freezetheworld more precise
exitsyscallfast checks for freezetheworld, but does so only by
checking if stopwait is positive. This can also happen during
stoptheworld, which is harmless, but confusing. Shortly, it will be
important that we get to the p.status cas even if stopwait is set.
Hence, make this test more specific so it only triggers with
freezetheworld and not other uses of stopwait.