As the static backtrace state pointer can be accessed from multiple
threads, use atomics to access it.
Regtested on x86_64-pc-linux-gnu.
libgfortran/ChangeLog:
2018-12-02 Janne Blomqvist <jb@gcc.gnu.org>
PR libfortran/88137
* runtime/backtrace.c (show_backtrace): Use atomic load/store to
access the static lbstate pointer.
From-SVN: r266724
df996c3fd2