[PATCH] Add V8FP Subtarget feature

[Tim Northover]

Hi Joey,

> This adds a V8FP subtarget feature to the ARM backend, and emits the eabi attributes for it.
> It also emits the new v8 NEON attribute.

Looks good (interesting changes to come, I think!), but can you add a
link to the ABI Addenda in docs/CompilerWriterInfo.rst when you
commit?

Cheers.

Tim.