malloc: Use assert.h's assert macro

malloc: Use assert.h's assert macro

This avoids assert definition conflicts if some of the headers used by
malloc.c happens to include assert.h.  Malloc still needs a malloc-avoiding
implementation, which we get by redirecting __assert_fail to malloc's
__malloc_assert.

	* malloc/malloc.c: Include <assert.h>.
	(assert): Do not define.
	[!defined NDEBUG] (__assert_fail): Define to __malloc_assert.