Initialize backtrace state once

From backtrace.h for backtrace_create_state:

   Calling this function allocates resources that can not be freed.
   There is no backtrace_free_state function.  The state is used to
   cache information that is expensive to recompute.  Programs are
   expected to call this function at most once and to save the return
   value for all later calls to backtrace functions.

So instead of calling backtrace_create_state every time we wish to
show a backtrace, do it once and store the result in a static
variable.  libbacktrace allows multiple threads to access the state,
so no need to use TLS.

Regtested on x86_64-pc-linux-gnu.

libgfortran/ChangeLog:

2018-11-30  Janne Blomqvist  <jb@gcc.gnu.org>

	PR libfortran/88137
	* runtime/backtrace.c (show_backtrace): Make lbstate a static
	variable, initialize once.

From-SVN: r266677

libgfortran/ChangeLog

@@ -1,3 +1,9 @@
+2018-11-30  Janne Blomqvist  <jb@gcc.gnu.org>
+
+	PR libfortran/88137
+	* runtime/backtrace.c (show_backtrace): Make lbstate a static
+	variable, initialize once.
+
 2018-11-23  Janne Blomqvist  <jb@gcc.gnu.org>
 
 	* runtime/error.c (MAGIC): Remove.

libgfortran/runtime/backtrace.c

@@ -146,11 +146,15 @@ full_callback (void *data, uintptr_t pc, const char *filename,
 void
 show_backtrace (bool in_signal_handler)
 {
-  struct backtrace_state *lbstate;
+  /* Note that libbacktrace allows the state to be accessed from
+     multiple threads, so we don't need to use a TLS variable for the
+     state here.  */
+  static struct backtrace_state *lbstate;
   struct mystate state = { 0, false, in_signal_handler };
- 
-  lbstate = backtrace_create_state (NULL, __gthread_active_p (),
-				    error_callback, NULL);
+
+  if (!lbstate)
+    lbstate = backtrace_create_state (NULL, __gthread_active_p (),
+				      error_callback, NULL);
 
   if (lbstate == NULL)
     return;