[gdb/testsuite] Use unique portnum in parallel testing

When instrumenting get_portnum using:
...
puts "PORTNUM: $res"
...
and running:
...
$ cd build/gdb
$ make check-parallel -j2 TESTS=gdb.server/*.exp
...
we run into:
...
Running gdb.server/abspath.exp ...
PORTNUM: 2345
...
and:
...
Running gdb.server/bkpt-other-inferior.exp ...
PORTNUM: 2345
...

This is because the test-cases are run in independent runtest invocations.

Fix this by handling the parallel case in get_portnum using:
- a file $objdir/cache/portnum to keep the portnum variable, and
- a file $objdir/cache/portnum.lock to serialize access to it.

Tested on aarch64-linux.

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

gdb/testsuite/lib/gdbserver-support.exp

@@ -141,6 +141,9 @@ proc get_portnum {} {
     # starting at $initial_portnum, to avoid conflicts with hung ports.
     set initial_portnum 2345
 
+    if { ![info exists ::GDB_PARALLEL] } {
+	# Sequential case.
+
 	# Currently available port number.
 	gdb_persistent_global portnum
 
@@ -153,6 +156,32 @@ proc get_portnum {} {
 	set res $portnum
 	incr portnum
 	return $res
+    }
+
+    # Parallel case.
+    with_lock portnum.lock {
+	# Keep portnum file alongside the lock that guards it.
+	set portnum_file [lock_dir]/portnum
+
+	if { [file exists $portnum_file] } {
+	    set fd [open $portnum_file r]
+	    set portnum [read $fd]
+	    close $fd
+
+	    set portnum [string trim $portnum]
+	} else {
+	    # Initialize.
+	    set portnum $initial_portnum
+	}
+
+	set next_portnum [expr $portnum + 1]
+
+	set fd [open $portnum_file w]
+	puts $fd $next_portnum
+	close $fd
+    }
+
+    return $portnum
 }
 
 # Locate the gdbserver binary.  Returns "" if gdbserver could not be found.