[Bug target/69548] libatomic fails to build with -Os on powerpc64-linux

[amodra at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69548

--- Comment #1 from Alan Modra <amodra at gmail dot com> ---
Created attachment 37516
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37516&action=edit
possible fix

This fixes the testcase on gcc-5.  I haven't yet bootstrapped it, or
investigated why the problem doesn't seem to occur on master.