[gdb/testsuite] Fix failure in gdb.threads/signal-sigtrap.exp

The test-case gdb.threads/signal-sigtrap.exp:
- installs a signal handler called sigtrap_handler for SIGTRAP,
- sets a breakpoint on sigtrap_handler, and
- expects the breakpoint to trigger after issuing "signal SIGTRAP".

Usually, that happens indeed:
...
(gdb) signal SIGTRAP^M
Continuing with signal SIGTRAP.^M
^M
Thread 1 "signal-sigtrap" hit Breakpoint 2, sigtrap_handler (sig=5)^M
28      }^M
(gdb) PASS: $exp: sigtrap thread 1: signal SIGTRAP reaches handler
...

Occasionally, I run into this failure on openSUSE Tumbleweed:
...
(gdb) signal SIGTRAP^M
Continuing with signal SIGTRAP.^M
^M
Thread 1 "signal-sigtrap" received signal SIGTRAP, Trace/breakpoint trap.^M
__pthread_create_2_1 () at pthread_create.c:843^M
(gdb) FAIL: $exp: sigtrap thread 1: signal SIGTRAP reaches handler
...

AFAIU, the problem is in the situation that is setup before issuing that
command, by running to a breakpoint in thread_function:
...
void *thread_function (void *arg) {
  return NULL;
}
int main (void) {
  pthread_t child_thread;
  signal (SIGTRAP, sigtrap_handler);
  pthread_create (&child_thread, NULL, thread_function, NULL);
  pthread_join (child_thread, NULL);
  return 0;
}
...

In the passing case, thread 2 is stopped in thread_function, and thread 1 is
stopped somewhere in pthread_join:
...
(gdb) info threads^M
  Id   Target Id                                          Frame ^M
  1    Thread ... (LWP ...) "signal-sigtrap" __futex_abstimed_wait_common64 ()
* 2    Thread ... (LWP ...) "signal-sigtrap" thread_function ()
...

In the failing case, thread 2 is stopped in thread_function, but thread 1 is
stopped somewhere in pthread_create:
...
(gdb) info threads^M
  Id   Target Id                                          Frame ^M
  1    Thread ... (LWP ...) "signal-sigtrap" __GI___clone3 ()
* 2    Thread ... (LWP ...) "signal-sigtrap" thread_function ()
...

What I think happens is that pthread_create blocks SIGTRAP at some point, and
if the "signal SIGTRAP" command is issued while that is the case, the signal
becomes pending and consequently there's no longer a guarantee that the signal
will be delivered to the inferior.

Instead the signal will be handled by gdb like this:
...
(gdb) info signals SIGTRAP
Signal        Stop	Print	Pass to program	Description
SIGTRAP       Yes	Yes	No		Trace/breakpoint trap
...

Fix this by adding a barrier that ensures that pthread_create is done before
we issue the "signal SIGTRAP" command.

Likewise in test-case gdb.threads/signal-command-handle-nopass.exp.

Using the fixed test-case, I tested my theory by explicitly blocking SIGTRAP:
...
+  sigset_t old_ss, new_ss;
+  sigemptyset (&new_ss);
+  sigaddset (&new_ss, SIGTRAP);
+  sigprocmask (SIG_BLOCK, &new_ss, &old_ss);
+
   /* Make sure that pthread_create is done once the breakpoint on
      thread_function triggers.  */
   pthread_barrier_wait (&barrier);

   pthread_join (child_thread, NULL);
+  sigprocmask (SIG_SETMASK, &old_ss, NULL);
...
and managed to reproduce the same failure:
...
(gdb) signal SIGTRAP^M
Continuing with signal SIGTRAP.^M
[Thread 0x7ffff7c00700 (LWP 13254) exited]^M
^M
Thread 1 "signal-sigtrap" received signal SIGTRAP, Trace/breakpoint trap.^M
0x00007ffff7c80056 in __GI___sigprocmask () sigprocmask.c:39^M
(gdb) FAIL: $exp: sigtrap thread 1: signal SIGTRAP reaches handler
...

Tested on x86_64-linux.

PR testsuite/26867
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26867

gdb/testsuite/gdb.threads/signal-command-handle-nopass.c

@@ -21,6 +21,8 @@
 #include <pthread.h>
 #include <signal.h>
 
+static pthread_barrier_t barrier;
+
 void
 handler (int sig)
 {
@@ -35,6 +37,13 @@ thread_function (void *arg)
     usleep (1);
 }
 
+void *
+thread_function_sync (void *arg)
+{
+  pthread_barrier_wait (&barrier);
+  return thread_function (arg);
+}
+
 int
 main (void)
 {
@@ -42,7 +51,15 @@ main (void)
   int i;
 
   signal (SIGUSR1, handler);
-  pthread_create (&child_thread, NULL, thread_function, NULL);
+
+  pthread_barrier_init (&barrier, NULL, 2);
+
+  pthread_create (&child_thread, NULL, thread_function_sync, NULL);
+
+  /* Make sure that pthread_create is done once the breakpoint on
+     thread_function triggers.  */
+  pthread_barrier_wait (&barrier);
+
   pthread_join (child_thread, NULL);
 
   return 0;

gdb/testsuite/gdb.threads/signal-sigtrap.c

@@ -20,6 +20,8 @@
 #include <pthread.h>
 #include <signal.h>
 
+static pthread_barrier_t barrier;
+
 void
 sigtrap_handler (int sig)
 {
@@ -31,6 +33,13 @@ thread_function (void *arg)
   return NULL;
 }
 
+void *
+thread_function_sync (void *arg)
+{
+  pthread_barrier_wait (&barrier);
+  return thread_function (arg);
+}
+
 int
 main (void)
 {
@@ -38,7 +47,13 @@ main (void)
 
   signal (SIGTRAP, sigtrap_handler);
 
-  pthread_create (&child_thread, NULL, thread_function, NULL);
+  pthread_barrier_init (&barrier, NULL, 2);
+
+  pthread_create (&child_thread, NULL, thread_function_sync, NULL);
+
+  /* Make sure that pthread_create is done once the breakpoint on
+     thread_function triggers.  */
+  pthread_barrier_wait (&barrier);
 
   pthread_join (child_thread, NULL);