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/docs/porting-guide.md b/docs/porting-guide.md
index cdb722a..687781a 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -85,7 +85,7 @@
 possible for the firmware to place variables in it using the following C code
 directive:
 
-    __attribute__ ((section("bakery_lock")))
+    __section("bakery_lock")
 
 Or alternatively the following assembler code directive: