Arm Cortex-A77 Support Upstreamed Finally To LLVM Clang 11

While the Arm Cortex-A77 was announced last year and already has been succeeded by the Cortex-A78 announcement, support for the A77 has finally been upstreamed to the LLVM Clang compiler.

The Cortex-A77 support was added to the GCC compiler last year while seemingly as an oversight the A77 support wasn't added to LLVM/Clang until this week.

This commit adds the A77 target and exposes it as a cortex-a77 CPU option. Targeting the A77 flips on ARMv8.2-A instructions along with FP16, SSBS, RCPC, CRC, and the dot product extension.

This A77 support will be part of LLVM Clang 11 due out around September.