Add arm64 build target.

Fix the weird blueprint file scheme. multilib is not necessary to build
multiple libraries for 64 bit targets.

Test: "mma -j128" built external/libffi for
aosp_(x86,x86_64,arm,arm64)-eng.
Change-Id: Ic102141e5768aaa8750f50c98e0be729553675aa
4 files changed