[libc] allow customizing filename given to __FC_assert in assert.h

useful to obtain reproducible oracles even with fully normalized input files

share/libc/assert.h

@@ -42,5 +42,12 @@ __POP_FC_STDLIB
 #ifdef NDEBUG 
 #define assert(ignore) ((void)0)
 #else
-#define assert(e) (__FC_assert((e) != 0,__FILE__,__LINE__,#e))
+#ifndef __FC_ASSERT_FILE__
+#define __FC_FILENAME__ __FILE__
+#else
+#define str(a) # a
+#define xstr(a) str(a)
+#define __FC_FILENAME__ xstr(__FC_ASSERT_FILE__)
+#endif
+#define assert(e) (__FC_assert((e) != 0,__FC_FILENAME__,__LINE__,#e))
 #endif