libc: Introduce __errordecl()

Define __errordecl and replace __attribute__((__error__("foo")))
with __errordecl. Make sure __errordecl is a no-op on clang, as it
generates a compile time warning.

Change-Id: Ifa1a2d3afd6881de9d479fc2adac6737871a2949

libc/include/sys/cdefs.h

@@ -330,6 +330,12 @@
 #define __wur
 #endif
 
+#if __GNUC_PREREQ__(4, 3)
+#define __errordecl(name, msg) extern void name(void) __attribute__((__error__(msg)))
+#else
+#define __errordecl(name, msg) extern void name(void)
+#endif
+
 /*
  * Macros for manipulating "link sets".  Link sets are arrays of pointers
  * to objects, which are gathered up by the linker.

libc/include/sys/stat.h

@@ -134,8 +134,7 @@ extern mode_t umask(mode_t);
 extern mode_t __umask_chk(mode_t);
 extern mode_t __umask_real(mode_t)
     __asm__(__USER_LABEL_PREFIX__ "umask");
-extern void __umask_error()
-    __attribute__((__error__("umask called with invalid mode")));
+__errordecl(__umask_error, "umask called with invalid mode");
 
 __BIONIC_FORTIFY_INLINE
 mode_t umask(mode_t mode) {