Make bilinearfilter_arm.c compiled only when HAVE_ARMV6, as its definitions
are v6 only. This is normally not a problem for static builds as the file
is elided at link time, but this was not being done properly for the
--enable-shared --enable-pic build.

Change-Id: Ic800a7cde751f74f22555c5b247f99f9df5e550d