fix multi-arch-exec for parallel mode

fix multi-arch-exec for parallel mode

I noticed today that multi-arch-exec.exp was failing in parallel mode.

The bug is that multi-arch-exec.c assumes the non-parallel directory
layout.

This patch fixes the problem using the same "BASEDIR" approach used in
other tests.

Tested both ways on x86-64 Fedora 18.
I'm checking this in.

2013-11-13  Tom Tromey  <tromey@redhat.com>

	* gdb.multi/multi-arch-exec.exp: Define BASEDIR when compiling.
	* gdb.multi/multi-arch-exec.c (main): Use BASEDIR.