More processors support under AARch64 state for auto detection.

Hi

I have added support for some additional processors (auto detection using -mcpu=native) under AArch64 state. Here are the changes:

1. Adds support for Applied Micro (APM) X-Gene-1 processor.
    - Auto detection (-mcpu=native) under AArch64 state
    - Correct set of pre-defined macros for the features of this model
    - Currently uses "generic" scheduler. Will be enhanced soon.
2. Adds support for ARM Cortex-A53, A57, A72 and A73
    - Auto detection (-mcpu=natie) under AArch64 state

Here is the Differential link: https://reviews.llvm.org/D24661

Note that, this is mainly for APM X-Gene-1 support. In addition, I have also added a couple of other processors from ARM Ltd. And for X-Gene-1, the current changes are only basic and there is no scheduler model added right now. We may add it later after proper testing; but that should not be a blocker for these changes, I believe.

Thanks a lot