[gdb/testsuite] Use unique portnum in parallel testing (check//% case)

Make target check//% is the gdb variant of a similar gcc make target [1].

When running tests using check//%:
...
$ cd build/gdb
$ make check//unix/{-fPIE/-pie,-fno-PIE/-no-pie} -j2 TESTS=gdb.server/*.exp
...
we get:
...
$ cat build/gdb/testsuite.unix.-fPIE.-pie/cache/portnum
2427
$ cat build/gdb/testsuite.unix.-fno-PIE.-no-pie/cache/portnum
2423
...

The problem is that there are two portnum files used in parallel.

Fix this by:
- creating a common lockdir build/gdb/testsuite.lockdir for make target
  check//%,
- passing this down to the runtests invocations using variable GDB_LOCK_DIR,
  and
- using GDB_LOCK_DIR in lock_dir.

Tested on aarch64-linux.

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

PR testsuite/31632
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31632

[1] https://gcc.gnu.org/install/test.html

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