Remove PREFIXED in specific.c/h; mark "specific" functions as GC_INNER

Remove PREFIXED in specific.c/h; mark "specific" functions as GC_INNER

* include/private/specific.h (PREFIXED): Remove.
* include/private/specific.h (key_t, key_create, setspecific,
remove_specific, slow_getspecific, getspecific): Expand PREFIXED macro
(add "GC_" prefix).
* specific.c (key_create, setspecific, remove_specific,
slow_getspecific, check_tsd_marks): Likewise.
* include/private/specific.h (GC_key_create, GC_setspecific,
GC_remove_specific, GC_slow_getspecific): Use GC_INNER for function.
* specific.c (GC_key_create, GC_setspecific, GC_remove_specific,
GC_slow_getspecific): Likewise.