Add "PACBTI" to -mcpu flag in cbuildgen

[AlbertHuang-CPU]

PACBTI are introduced to v8.1m CPU such as CM85, CM52, SMC3.
They are enabled by default with these CPU specified in -mcpu flag.
We need to address the requirement for both sides of the coin.
1, to actually enable PACBTI, use -mbranch-protection and --library_security=protection, and this is reflected by CMake variable BRANCHPROT.
2, to disable(exclude) PACBTI, the compiler provide "+nopacbti" option for the -mcpu flag. There is currently no implementation for explicit excluding it.

Background information:
This is requested by CM52 user. For those customers who don't need PACBTI, they want to make it clear when they don't include PACBTI in HW the SW(toolchain) will also exclude it. Otherwise there will be confusion, e.g. the uVision will display "-mcpu=cortex-m52+pacbti" by default.
The queary above triggered AC upgrade from
https://developer.arm.com/documentation/109443/6-22-1LTS/armclang-Reference/Other-Compiler-specific-Features/Supported-architecture-feature-combinations-for-specific-processors?lang=en
to
https://developer.arm.com/documentation/109443/6-22-2LTS/armclang-Reference/Other-Compiler-specific-Features/Supported-architecture-feature-combinations-for-specific-processors?lang=en

[jkrech]

processor feature from pdsc Dpacbti
and
processor from YAML input branch-protection:

Require intersection check to allow branch-protection settings for devices that have a processor implementing PACBTI.