cpu: Only activate thread 0 in Minor if the CPU is active

cpu: Only activate thread 0 in Minor if the CPU is active

Minor currently activates thread 0 in startup() to work around an
issue where activateContext() is called from LiveProcess before the
process entry point is known. When activateContext() is called, Minor
creates a branch instruction to the process's entry point. The first
time it is called, the branch points to an undefined location (0). The
call in startup() updates the branch to point to the actual entry
point.

When instantiating a switched out Minor CPU, it still tries to
activate thread 0. This is clearly incorrect since a switched out CPU
can't have any active threads. This changeset adds a check to ensure
that the thread is active before reactivating it.