x86, vdso, build: Don't rebuild 32-bit vdsos on every make

vdso32/vclock_gettime.o was confusing kbuild.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Stefani Seibold <stefani@seibold.net>
Link: http://lkml.kernel.org/r/d741449340642213744dd659471a35bb970a0c4c.1395789923.git.luto@amacapital.net


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