[aarch64] Compile libcxx with PAuth and BTI

This change adds -mbranch-protection flag to enable
building libcxx with Armv8.3-A Pointer Authentication and
Armv8.5-A Branch Target Identification for NDK.

Instructions emitted for these extensions are all in the hint
space which are executed as NOPs on devices not supporting
any of these features.

Change-Id: Ib50756022887e9a2f783651e0c3f0ec18a36abbb