[AArch64] Add support for Cortex-A76 and Cortex-A76AE

- Add LLVM backend support for Cortex-A76 and Cortex-A76AE
- Documentation can be found at
  https://developer.arm.com/products/processors/cortex-a/cortex-a76

Differential Revision: https://reviews.llvm.org/D57764

llvm-svn: 354789