1.0.23.7: introduce page type flags

1.0.23.7: introduce page type flags

 (Code and data separation 2/5.)

 * Use page_type_flag instead of unboxed/boxed booleans in
   gc_find_freeish_pages, gc_alloc_new_region, gc_alloc_large,
   gc_alloc_with_region, and gc_alloc_update_page_tables.

 * Similarly use page_type_flag instead of unboxed/boxed boolean in
   gc_general_alloc. Also make it static inline on GENCGC platforms,
   and move to gc-internal.h (since the page_type_flag is constant in
   every call by inlining we get rid of the branch to select the
   region, whoopee.)