Fix visibility of GC_push_all/conditional, GC_push_other_roots symbols

Fix visibility of GC_push_all/conditional, GC_push_other_roots symbols

* include/private/gc_priv.h (GC_push_all, GC_push_other_roots):
Declare as GC_API_PRIV (instead of GC_INNER) to make the symbol
externally visible to some well-known 3rd-party software (e.g., ECL).
* include/private/gc_priv.h (GC_push_conditional): Declare as
GC_API_PRIV (only if GC_DISABLE_INCREMENTAL is undefined).
* mark.c (GC_push_all, GC_push_conditional): Remove GC_INNER (to match
the declaration).
* os_dep.c (GC_push_other_roots): Likewise.