fix argv0-symlink.exp for parallel mode

fix argv0-symlink.exp for parallel mode

argv0-symlink.exp doesn't work properly if standard_output_file puts
files into a per-test subdirectory.  That's because it assumes that
files appear in $subdir, which is no longer true.

This patch fixes the problem by computing the correct directory at
runtime.

Tested both with and without GDB_PARALLEL on x86-64 Fedora 18.

2013-11-04  Tom Tromey  <tromey@redhat.com>

	* gdb.base/argv0-symlink.exp: Compute executable's directory
	dynamically.