Replace SIG_SUSPEND macro to a variable in pthread_stop_world

Replace SIG_SUSPEND macro to a variable in pthread_stop_world

* misc.c (GC_get_suspend_signal): Define (as always returning -1) in
this file only if Darwin or Win32 threads.
* os_dep.c (GC_dirty_init): Replace SIG_SUSPEND with the value of
GC_get_suspend_signal() invocation.
* pthread_support.c (pthread_sigmask): Likewise.
* pthread_stop_world.c (GC_sig_suspend): New STATIC variable (only
unless NaCl or OpenBSD) initialized to SIG_SUSPEND.
* pthread_stop_world.c (GC_unblock_gc_signals,
GC_suspend_handler_inner, GC_suspend_all, GC_stop_init): Use
GC_sig_suspend instead of SIG_SUSPEND (only unless NaCl or OpenBSD);
update comment.
* pthread_stop_world.c (GC_get_suspend_signal): Define function (which
returns GC_sig_suspend if available).