openssl: enable ARM assembly acceleration

openssl: enable ARM assembly acceleration

Tested myself on ixp4xx and mvebu, and (originally)
by Daniel on i.MX6. Also tested on a MIPS target,
to make sure the change to ASFLAGS does not break things.

Based on a patch submitted by Daniel Drown:

https://lists.openwrt.org/pipermail/openwrt-devel/2014-July/026639.html

Signed-off-by: Claudio Leite <leitec@staticky.com>
Signed-off-by: Daniel Drown <dan-openwrt@drown.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44618 3c298f89-4303-0410-b956-a3cf2f4a3e73