[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 b4f76713
("Fix gdb.base/align-*.exp and Clang + LTO and AIX GCC"), by incrementing the
variable.

Tested on x86_64-linux with gcc and clang.