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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174369 91177308-0d34-0410-b5e6-96231b3b80d8