Remove cyclic dependency in AArch64 libraries

This moves the bit twiddling and string fiddling functions required by other
parts of the backend into a separate library. Previously they resided in
AArch64Desc, which created a circular dependency between various components.
Fix formatting in AArch64 backend.

This should fix three purely whitespace issues:
    + 80 column violations.
    + Tab characters.
    + TableGen brace placement.

No functional changes.