Avoid explicit use of machine-specific x_THREAD_STATE macros (Darwin)
(code refactoring)

* include/private/gc_priv.h (GC_MACH_THREAD_STATE,
GC_MACH_THREAD_STATE_COUNT): Remove explicit definition to
PPC_THREAD_STATE[_COUNT], ARM_THREAD_STATE[_COUNT] (as the correct
values are defined via MACHINE_THREAD_STATE[_COUNT] macro).