[gdb/testsuite] Fix gdb.threads/access-mem-running-thread-exit.exp with clang

When running test-case gdb.threads/access-mem-running-thread-exit.exp with
clang, we run into:
...
(gdb) print global_var = 555^M
No symbol "global_var" in current context.^M
(gdb) FAIL: gdb.threads/access-mem-running-thread-exit.exp: all-stop: \
  access mem (write to global_var, inf=2, iter=1)
...

The problem is that clang removes the unused variable.

Fix this in the same way as done in commit b4f767131f
("Fix gdb.base/align-*.exp and Clang + LTO and AIX GCC"), by incrementing the
variable.

Tested on x86_64-linux with gcc and clang.

gdb/testsuite/gdb.threads/access-mem-running-thread-exit.c

@@ -102,6 +102,8 @@ main (void)
 {
   int i;
 
+  global_var++;
+
   for (i = 0; i < 4; i++)
     {
       struct thread_arg *p;