GCS (Guarded Control Stack) support is missing

[ggardet]

When building with GCC15+, binutils 2.44+, glibc 2.41+ and -mbranch-protection=standard on (openSUSE Tumbleweed) aarch64, this enables PAC, BTI and GCS (Guarded Control Stack).
PAC and BTI are already supported with #312
A patch similar to smuellerDD/leancrypto@fe9751f for src/asm/*_arm64_aapcs_elf_gas.S should be enough if assembler code does nothing around control flow (things like loaders, exception unwinders, etc)

More details on GCS:

• https://community.arm.com/arm-community-blogs/b/tools-software-ides-blog/posts/gcc-15-continuously-improving#guarded
• https://docs.kernel.org/next/arch/arm64/gcs.html

[olk]

please provide a PR