external/openssl: add support for multilib builds

Support multilib builds by setting LOCAL_*_arch instead of setting
LOCAL_* based on $(TARGET_ARCH).  Also converts the makefile
included once per file to be separate target and host makefiles
included once per build rule.

This is a revert of the revert in 92141a02,
with conflicts resolved and a build fix from Fengwei Yin
<fengwei.yin@intel.com> to remove the unset local_cflags variable.

Change-Id: I77c786b69e946bb82b5b8613de99cf4b3fa6e30a