[arm64] Build NDK with PAuth and BTI

This change adds a new arch named "armv8-a-branchprot". The new target
is no different from the usual "armv8-a" variant except it has the
compiler flag "-mbranch-protection=standard" set.

With this flag compiler emits Armv8.3-A Pointer Authentication and
Armv8.5-A Branch Target Identification instructions. All of these
instructions are in the hint space; therefore devices which do not
support any of these extensions would execute them as NOPs.

In terms of directory structure there is no change in the output of
build/soong/scripts/build-ndk-prebuilts.sh script.

Test: build NDK prebuilts

Change-Id: Ie52ccba5653fbf567f0b7ca312d8a8e71602ad06