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
5 files changed