[clang][AARCH64] Add __ARM_FEATURE_{PAC, BTI}_DEFAULT defines

Summary:
As defined by Arm C Language Extensions (ACLE) these macro defines
should be set to specific values depending on -mbranch-protection.

Reviewers: chill

Reviewed By: chill

Subscribers: danielkiss, cfe-commits, kristof.beyls

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77134