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.
Fix typo in comment of GC_mark_from

* mark.c (GC_mark_from): Fix typo (capitalization) in comment ("ASAP").
Eliminate GC_first_nonempty atomic value reload in GC_mark_local assertion

* mark.c (GC_mark_from, GC_mark_local, GC_do_parallel_mark): Reformat
code partially.
* mark.c (GC_mark_local): Use "my_first_nonempty" in assertion
instead of reloading value of GC_first_nonempty twice.
Fix assertion violation in GC_mark_from prefetch loop

* mark.c (GC_mark_from): If "descr" is less than size of word then
skip prefetching and PUSH_CONTENTS of the current entry (thus,
eliminate assertion violation in the prefetch loop) if SMALL_CONFIG is
not defined.
TODO file: Add pending TODO/FIXME items
Fix stackbottom/stack_end assignment in GC_call_with_gc_active

* misc.c (GC_call_with_gc_active): Add GC_noop1 call between "fn" call
and "return" statement to discourage the compiler from treating the
function call as a tail one (thus ensuring "stackbottom" COOLER_THAN
any stack pointer value obtained inside "fn").
* pthread_support.c (GC_call_with_gc_active): Likewise.
* win32_threads.c (GC_call_with_gc_active): Likewise.
Fix 'stack section' pointer passed to push_all_stack_sections (Pthreads)

* pthread_stop_world.c (GC_push_all_stacks): Use
traced_stack_sect->prev instead of traced_stack_sect if saved_stack_ptr
(of traced_stack_sect) is the same as stop_info.stack_ptr of the
examined thread (thus preventing assertion violation in
GC_push_all_stack_sections invoked during incremental collection).