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
diff --git a/libc/include/sys/stat.h b/libc/include/sys/stat.h
index 4e8beb6..da49e1a 100644
--- a/libc/include/sys/stat.h
+++ b/libc/include/sys/stat.h
@@ -134,8 +134,7 @@
 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) {