Collapse multiple page_was_dirty, remove_protection, read_dirty definitions

Collapse multiple page_was_dirty, remove_protection, read_dirty definitions
(code refactoring)

* os_dep.c [NEED_PROC_MAPS] (READ): Move macro definition inside the
function where it is used; undefine the macro at the function end.
* os_dep.c [PROC_VDB] (READ): Likewise.
* os_dep.c [!GWW_VDB] (GC_GWW_AVAILABLE): Define to FALSE.
* os_dep.c [!GC_DISABLE_INCREMENTAL] (GC_page_was_dirty,
GC_page_was_ever_dirty, GC_remove_protection, GC_read_dirty): Collapse
multiple definitions into a single one; reformat code; replace
GC_ATTR_UNUSED with (void)arg.
* os_dep.c [GWW_VDB && MPROTECT_VDB] (GC_gww_page_was_ever_dirty):
Remove.
* os_dep.c [GWW_VDB && !MPROTECT_VDB] (GC_read_dirty): Rename to
GC_gww_read_dirty; define as GC_INLINE.
* os_dep.c [PROC_VDB] (GC_read_dirty): Rename to GC_proc_read_dirty;
define as GC_INLINE.