Eliminate 'label not used' cppcheck false warnings in GC_mark_X

Eliminate 'label not used' cppcheck false warnings in GC_mark_X
(code refactoring of PUSH_CONTENTS[_HDR] to eliminate 'goto' statement)

* include/private/gc_hdrs.h (HC_GET_HDR): Remove exit_label argument;
replace goto with break; remove do-while(0) (as break is now used to
pass control to some place of the caller).
* include/private/gc_pmark.h (SET_MARK_BIT_EXIT_IF_SET): Likewise.
* include/private/gc_pmark.h [!USE_MARK_BYTES] (OR_WORD_EXIT_IF_SET):
Likewise.
* include/private/gc_pmark.h (PUSH_CONTENTS, PUSH_CONTENTS_HDR):
Remove exit_label argument (and the comment about it).
* include/private/gc_pmark.h (SET_MARK_BIT_EXIT_IF_SET): Refine comment
about the exit.
* mark.c (GC_mark_from): Remove PUSH_CONTENTS exit<N> argument.
* typd_mlc.c (GC_typed_mark_proc): Likewise.
* mark.c (GC_mark_and_push, GC_mark_and_push_stack): Remove
PUSH_CONTENTS_HDR label argument and the label itself.