x86: Install unstripped copy of 64bit vdso to disk

This keeps an unstripped copy of the 64bit vDSO images built before they are
stripped and embedded in the kernel.  The unstripped copies get installed
in $(MODLIB)/vdso/ by "make install" (or you can explicitly use the
subtarget "make vdso_install").  These files can be useful when they
contain source-level debugging information.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>