[AArch64][NFC] Refactor the tail-folding option

This patch does simple refactoring of the tail-folding
option in preparation for enabling tail-folding by
default for neoverse-v1. It adds a default tail-folding
option field to the AArch64Subtarget class that can
be set on a per-CPU.

Differential Revision: https://reviews.llvm.org/D149659