Allow to compile GC_inner_start_routine aside from extra/gc.c

Allow to compile GC_inner_start_routine aside from extra/gc.c
(for proper exceptions support on Linux)

* extra/gc.c: Do not include pthread_start.c if
GC_PTHREAD_START_STANDALONE defined (thus pthread_start.c could go to
a standalone compilation unit).
* include/private/pthread_support.h (GC_INNER_PTHRSTART): New macro
(defined to either empty or GC_INNER depending on
GC_PTHREAD_START_STANDALONE).
* include/private/pthread_support.h (GC_start_rtn_prepare_thread,
GC_thread_exit_proc): Replace GC_INNER to GC_INNER_PTHRSTART.
* pthread_support.c (GC_thread_exit_proc,
GC_start_rtn_prepare_thread): Likewise.
* pthread_start.c (GC_inner_start_routine): Define as
GC_INNER_PTHRSTART; do not call GC_log_printf if
GC_PTHREAD_START_STANDALONE (since the function might be visible only
within extra/gc.c compilation unit).
* pthread_support.c (GC_inner_start_routine): Declare as
GC_INNER_PTHRSTART.