Manually define the necessary HWCAP if sys/auxv.h didn't define
it. (It isn't defined in the Android NDK, one has to include
asm/hwcap.h to get definitions of them. Additionally, the aarch64
cross libc headers in Ubuntu 16.04 lacked HWCAP definitions from
sys/auxv.h, also present in asm/hwcap.h. Ubuntu 16.04's armhf
cross libc headers did however define HWCAP values in sys/auxv.h.)

Don't use any detection at all for NEON on aarch64, since it is a
mandatory feature there. On WSL on Windows 10 1803 arm64,
getauxval(AT_HWCAP) returns 0.