Need more synchronization around check+use of tid->PrivateInfo

There are a bunch of methods in class Thread that test tid->PrivateInfo and
then go off to use it, without synchronizing the access. It is theoretically possible
(with details varying by platform) that the thread that was just alive (its
tid->PrivateInfo was nonzero) could be killed before using PrivateInfo. If
that happened, PrivateInfo would be zeroed and the the system would probably
crash. This has not actually been observed.

A quick list of affected methods of class Thread are (approximately):

Thread.getPriority
Thread.setPriority
Thread.suspend
Thread.resume

Thread.isAlive should also be considered for synchronization and correctness,
although it doesn't suffer from the same race condition.

I don't think that Thread.start and Thread.stop are affected -- they are
synchronized in the C code.