arm: Improve use of getauxval in quirky environments

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.