[ARM][AArch64] Armv8.4-A Enablement

[ARM][AArch64] Armv8.4-A Enablement

Initial patch adding assembly support for Armv8.4-A.

Besides adding v8.4 as a supported architecture to the usual places, this also
adds target features for the different crypto algorithms. Armv8.4-A introduced
new crypto algorithms, made them optional, and allows different combinations:

- none of the v8.4 crypto functions are supported, which is independent of the
  implementation of the Armv8.0 SHA1 and SHA2 instructions.
- the v8.4 SHA512 and SHA3 support is implemented, in this case the Armv8.0
  SHA1 and SHA2 instructions must also be implemented.
- the v8.4 SM3 and SM4 support is implemented, which is independent of the
  implementation of the Armv8.0 SHA1 and SHA2 instructions.
- all of the v8.4 crypto functions are supported, in this case the Armv8.0 SHA1
  and SHA2 instructions must also be implemented.

The v8.4 crypto instructions are added to AArch64 only, and not AArch32,
and are made optional extensions to Armv8.2-A.

The user-facing Clang options will map on these new target features, their
naming will be compatible with GCC and added in follow-up patches.

The Armv8.4-A instruction sets can be downloaded here:
https://developer.arm.com/products/architecture/a-profile/exploration-tools

Differential Revision: https://reviews.llvm.org/D48625

llvm-svn: 335953