FEAT_BTI Support for AArch64

The implementation of the FEAT_BTI feature introduced in Armv8.5 will require changes to DR branching code, e.g. exception/signal handling, stitching together basic-blocks in the code cache etc.

Branch Target Identification (FEAT_BTI) allows memory pages to be guarded against the execution of instructions that are not the intended target of a branch, see https://developer.arm.com/documentation/ddi0596/2021-06/Base-Instructions/BTI--Branch-Target-Identification-

Neoverse V2 is expected to support FEAT_BTI.