[ARM][AArch64] Cortex-A75 and Cortex-A55 support

This patch introduces support for Cortex-A75 and Cortex-A55, Arm's
latest big.LITTLE A-class cores. They implement the ARMv8.2-A
architecture, including the cryptography and RAS extensions, plus
the optional dot product extension. They also implement the RCpc
AArch64 extension from ARMv8.3-A.

Cortex-A75:
https://developer.arm.com/products/processors/cortex-a/cortex-a75

Cortex-A55:
https://developer.arm.com/products/processors/cortex-a/cortex-a55

Differential Revision: https://reviews.llvm.org/D36667

llvm-svn: 311316