[ARM][libcxxabi] Add PACBTI-M support to libcxxabi

This change consists of just adding 'BTI' to the prologue of Arm assembly
functions, which is just the one: __cxa_end_cleanup

This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:

https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension

The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:

https://developer.arm.com/documentation/ddi0553/latest

The following people contributed to this patch:

- Mikhail Maltsev

Reviewed By: lenary, danielkiss

Differential Revision: https://reviews.llvm.org/D112432