arch: arm: cortex_m: Add support for PACBTI flags

Introduce a Kconfig choice to select Pointer Authentication and Branch
Target Identification (PACBTI) features on Armv8.1-M Mainline CPUs.
The available options map to supported `-mbranch-protection` levels
(`standard`, `pac-ret`, `bti`, and combinations) documented at
https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html


These options enable hardware-assisted control-flow integrity mechanisms on
targets like Cortex-M85, and require a toolchain with PACBTI support, such
as GCC 14.2 or newer.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>