Support x86+arm multilib build.

Support TARGET_2ND_ARCH as the binary translation arch.
See target/board/generic_x86_arm/BoardConfig.mk and
target/product/aosp_x86_arm.mk as example for the setup.

In BoardConfig, use the TARGET_2ND_ARCH/etc. variables to set up the
binary translation arch;
Set "TARGET_TRANSLATE_2ND_ARCH := true" to tell the build system it's
not a typical 64-bit multilib configuration.
In product makefile, use "PRODUCT_PACKAGES += libfoo_<2nd_arch>" to
install the TARGET_2ND_ARCH libraries. This also pulls in any dependency
libraries.
By default we don't install any TARGET_2ND_ARCH modules, unless it's
pulled in by PRODUCT_PACKAGES.

Bug: 27526885
Change-Id: I0578e9c80da0532d2fa886a8fcdb140bbc703009