Remove PUSH_CONTENTS_HDR multi-line macro

Remove PUSH_CONTENTS_HDR multi-line macro
(code refactoring)

* include/private/gc_pmark.h (PUSH_CONTENTS): Use GC_push_contents_hdr
instead of PUSH_CONTENTS_HDR.
* mark.c (GC_mark_and_push, GC_mark_and_push_stack): Likewise.
* include/private/gc_pmark.h (PUSH_CONTENTS_HDR): Replace macro
definitions with a unified inline function (GC_push_contents_hdr);
return updated value of mark_stack_top; remove unnecessary type casts
and parentheses.
* include/private/gc_priv.h [!KEEP_BACK_PTRS] (GC_STORE_BACK_PTR): Cast
source argument to void (to avoid "unused argument" compiler warning).