libiberty: fix build of gdb with clang This use a upstream fix at https://gcc.gnu.org/ml/gcc-patches/2008-07/msg00292.html BUG=475970 TEST=CC=x86_64-cros-linux-gnu-clang CXX=x86_64-cros-linux-gnu-clang++ emerge-falco gdb Change-Id: Ia46631625ad600adfe314ad6e36c154a10cfad4a