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

Add frontend tests 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/D36731

llvm-svn: 311319