gdb/testsuite: respect GDBSERVER variable in remote-stdio-gdbserver "board"

The comment on top of gdb/testsuite/boards/remote-stdio-gdbserver.exp says
that user can specify path to gdbserver on remote system by setting
GDBSERVER variable. However, this variable was ignored and /usr/bin/gdbserver
was used unconditionally.

This commit updates the code to respect GDBSERVER if set and fall back to
/usr/bin/gdbserver if not.