r193740 - [AArch64] Add some CPU targets for "generic", A-53 and A-57.

[Tim Northover]

> To me it would be odd if the AArch64 CLI behaved differently to AArch32,
> except where absolutely necessary.

The general GCC interface has been encouraging -march/-mtune for years
now. I've no idea what reasons 32-bit ARM chose to go against that,
but I'd want to make sure they were valid for AArch64 before
perpetuating it.

Inertia doesn't seem like a great reason except for ARM-only projects,
which are fairly rare.

Tim.