x86-64: Add --no-undefined to vDSO build

This gives much nicer diagnostics when something goes wrong.  It's
supported at least as far back as binutils 2.15.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/de0b50920469ff6359c529526e7639fdd36fa83c.1310563276.git.luto@mit.edu


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>