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