testsuite: Treat empty string in needs_status_wrapper as a false value

GCC testsuite treats empty string in [target_info needs_status_wrapper] the
same way as "0" - as a false value. GDB testsuite on the other hand recognizes
only zero as false. This patch changes this, to improve compatibilty of board files.

gdb/testsuite/ChangeLog:

2013-10-17  Anton Kolesov <Anton.Kolesov@synopsys.com>

    * lib/gdb.exp (gdb_compile, gdb_wrapper_init): Treat empty string in
    target_info needs_status_wrapper the same way as zero - as a false value.