build: Support PACBTI in AArch64 assembly
This adds support for the branch protection mechanism provided in 64-bit Arm architecture, namely:
- Armv8.3-A Pointer Authentication (PAC)
- Armv8.5-A Branch Target Identification (BTI)
Note that PAC is currently unused in the assembly files as they only define leaf functions wrapped by the corresponding C functions, which will be instrumented by the compiler.
Signed-off-by: Daiki Ueno ueno@gnu.org