CompilerThread should not be a Java thread

The CompilerThreads in HotSpot should not be Java threads. They end up in the
user-visible thread list in the main thread group and can therefore be operated
upon by Java code. This causes bad interactions with debuggers and operations
like Thread.stop(). It's likely that they were made Java threads in order to be
able to transition their state to _thread_in_native; they don't run any Java
bytecodes (or at least definitely should not.) We should make these threads non-
Java threads and fix any runtime assertions that come up in 1.4.2.