scripts/link-vmlinux.sh: only filter kernel symbols for arm

Actually CONFIG_PAGE_OFFSET isn't same with PAGE_OFFSET, so
it isn't easy to figue out PAGE_OFFSET defined in header
file from scripts.

Because CONFIG_PAGE_OFFSET may not be defined in some ARCHs(
64bit ARCH), or defined as bogus value in !MMU case, so
this patch only applys the filter on ARM when CONFIG_PAGE_OFFSET
is defined as the original problem is only on ARM.

Cc: <stable@vger.kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Fixes: f6537f2f
Singed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>