[gdb/testsuite] Factor out proc lock_dir

In lib/rocm.exp we have:
...
set gpu_lock_filename $objdir/gpu-parallel.lock
...

This decides both the lock file name and directory.

Factor out a new proc lock_dir that decides on the directory, leaving just:
...
set gpu_lock_filename gpu-parallel.lock
...

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>