Code refactoring (test single macro instead of MSWIN32/MSWINCE pair)

Code refactoring (test single macro instead of MSWIN32/MSWINCE pair)

* alloc.c (GC_stopped_mark): Test USE_WINALLOC macro instead of
MSWIN32 and MSWINCE.
* include/private/gc_priv.h (GC_add_current_malloc_heap): Likewise.
* os_dep.c (MMAP_SUPPORTED, SBRK_ARG_T, GC_mem_top_down, GC_unmap,
GC_remap, GC_unmap_gap, PROTECT, UNPROTECT): Likewise.
* include/private/gcconfig.h (USE_WINALLOC): New macro (defined if
MSWIN32 or MSWINCE).
* os_dep.c (GC_sysinfo): Define just before GC_setpagesize.
* os_dep.c (GC_malloc_heap_list, GC_is_malloc_heap_base,
GC_get_allocation_base, GC_add_current_malloc_heap, GC_is_heap_base):
Test USE_WINALLOC macro instead of CYGWIN32.
* os_dep.c (USE_GLOBAL_ALLOC): Define macro even if CYGWIN32 (to
remove redundant macro test).
* os_dep.c (GC_mem_top_down): Remove STATIC (as used for debugging);
define as macro in case of 0 value.