Do not cast pointer to size_t in GET_MEM definition

(refactoring)

Issue #627 (bdwgc).

* include/private/gcconfig.h [GC_PRIVATE_H && !OS2 && (NEXT || DOS4GW
|| NONSTOP || SOLARIS && !USE_MMAP || RTEMS || EMBOX || KOS
|| __CC_ARM)] (GET_MEM): Cast calloc() result to ptr_t instead of
size_t.

include/private/gcconfig.h

@@ -3436,8 +3436,8 @@ void *os2_alloc(size_t bytes);
 #    if defined(REDIRECT_MALLOC) && !defined(CPPCHECK)
 #      error Malloc redirection is unsupported
 #    endif
-#    define GET_MEM(bytes)                                          \
-      HBLKPTR((size_t)calloc(1, SIZET_SAT_ADD(bytes, GC_page_size)) \
+#    define GET_MEM(bytes)                                         \
+      HBLKPTR((ptr_t)calloc(1, SIZET_SAT_ADD(bytes, GC_page_size)) \
               + GC_page_size - 1)
 #  elif defined(MSWIN_XBOX1)
 ptr_t GC_durango_get_mem(size_t bytes);