[gdb/testsuite] Move gpu-parallel.lock to cache dir

The lock directory returned by lock_dir is currently $objdir.

It seems possible to leave a stale lock file that blocks progress in a
following run.

Fix this by using a directory that is guaranteed to be initially empty when
using GDB_PARALLEL, like temp or cache.

In gdb/testsuite/README I found:
...
cache in particular is used to share data across invocations of runtest
...
which seems appropriate, so let's use cache for this.

Tested on aarch64-linux.

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