Fix GC_finalizer_nested size to workaround alignment problem in Watcom

Fix GC_finalizer_nested size to workaround alignment problem in Watcom

* finalize.c (GC_finalizer_nested): Change type from char to int to
force GC symbols proper alignment for some compilers (e.g., Watcom);
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* finalize.c (GC_check_finalizer_nested): Cast GC_finalizer_nested
properly (since it holds an unsigned char value).
* os_dep.c (GC_get_maps): Remove static "init_buf" variable (of char
size) and initialize "maps_buf" to NULL since it is allocated anyway
(since maps_size is non-zero).
* os_dep.c (GC_register_data_segments): Remove static "dummy" variable
(of char size) and use GC_pages_executable instead.