aarch64: advertise assembler BTI support
As per the AArch64 ELF ABI, object files that support BTI need to advertise it via a dedicated Linux ELF object file property in an ELF note section. The linker can then check if all input object files support the feature to advertise it at the shared object or executable level.
In the case of BTI, this is necessary to enable the guarded page bit in the memory mapping for BTI-capable .text sections.
This defines a macro to populate the note section and uses it.