[libc] Add ASSERTED macro to test if any assertion has triggered

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/assert.h

 
 extern unsigned int assertion_failures;
 
+#define ASSERTED ( ASSERTING && ( assertion_failures != 0 ) )
+
 /** printf() for assertions
  *
  * This function exists so that the assert() macro can expand to