[AArch64] Add default features for CPUs on AArch64 target.

For ARM target, we can use CRYPTO and CRC features if we select
cortex-a57 by '-mcpu', but for AArch64 target, it doesn't work
unless adding with '-mfpu=crypto-neon-fp-armv8'. To keep consistency
between front-end and back-end and get end-users more easier to use,
we'd better add default feature for CPUs on AArch64 target as well.

llvm-svn: 210625