Remove direct usage of __attribute__((foo))

Migrate all direct usage of __attribute__ to usage of their
corresponding macros from cdefs.h.
e.g.:
 - __attribute__((unused)) -> __unused

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
diff --git a/drivers/io/io_semihosting.c b/drivers/io/io_semihosting.c
index 63d0f68..30ca99c 100644
--- a/drivers/io/io_semihosting.c
+++ b/drivers/io/io_semihosting.c
@@ -91,7 +91,7 @@
 
 
 /* Open a file on the semi-hosting device */
-static int sh_file_open(io_dev_info_t *dev_info __attribute__((unused)),
+static int sh_file_open(io_dev_info_t *dev_info __unused,
 		const uintptr_t spec, io_entity_t *entity)
 {
 	int result = -ENOENT;