add libc building to agbcc.

libc/stdlib/msize.c

@@ -0,0 +1,17 @@
+/* msize.c -- a wrapper for malloc_usable_size.  */
+
+#include <_ansi.h>
+#include <reent.h>
+#include <stdlib.h>
+#include <malloc.h>
+
+#ifndef _REENT_ONLY
+
+size_t
+_DEFUN (malloc_usable_size, (ptr),
+	_PTR ptr)
+{
+  return _malloc_usable_size_r (_REENT, ptr);
+}
+
+#endif