[gdb/testsuite] Make portnum a persistent global

When instrumenting get_portnum using:
...
    puts "PORTNUM: $res"
...
and running:
...
$ cd build/gdb
$ make check TESTS=gdb.server/*.exp
...
we get:
...
Running gdb.server/target-exec-file.exp ...
PORTNUM: 2345
Running gdb.server/stop-reply-no-thread-multi.exp ...
PORTNUM: 2345
PORTNUM: 2346
PORTNUM: 2347
PORTNUM: 2348
PORTNUM: 2349
PORTNUM: 2350
...

So, while get_portnum does return increasing numbers in a single test-case, it
restarts at each test-case.

This is a regression since the introduction of persistent globals.

Fix this by using "gdb_persistent_global portnum", such that we get:
...
Running gdb.server/target-exec-file.exp ...
PORTNUM: 2345
Running gdb.server/stop-reply-no-thread-multi.exp ...
PORTNUM: 2346
PORTNUM: 2347
PORTNUM: 2348
PORTNUM: 2349
PORTNUM: 2350
PORTNUM: 2351
...

Tested on aarch64-linux.

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