libpayload: Use __func__ instead of old __FUNCTION__

Change-Id: Ic3c22ac101a2ff44f97b2ac3fe3c0a89391718de
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72380
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

payloads/libpayload/include/assert.h

@@ -50,7 +50,7 @@ void mock_assert(const int result, const char *const expression, const char *con
 	if ((statement) == 0) {					\
 		fprintf(stderr, "assertion failed in file %s, "	\
 			"function %s(), line %d\n",		\
-			 __FILE__, __FUNCTION__, __LINE__);	\
+			 __FILE__, __func__, __LINE__);	\
 		abort();					\
 	}
 #endif