Start of support for Android on AArch64

The build does not get far before it runs into trouble in the GNU libstdc++
headers, though:

android-ndk-r10/sources/cxx-stl/gnu-libstdc++/4.9/libs/x86/include/bits/opt_random.h:33:23:
fatal error: x86intrin.h: No such file or directory

Change-Id: I9d459c64980091ba8bf5b3d631d47342625f6be9