AArch64: Cleanup PAC and BTI

AArch64: Cleanup PAC and BTI

Now we finally support modern GCC and binutils, it's time for a cleanup.
Use PAC and BTI instructions unconditionally and use proper assembler syntax.
Remove the PR target/94791 strip_pac workarounds for buggy GCCs.  Remove the
PAC/BTI configure checks - always emit GNU property notes on assembly files.
Change cfi_window_save to the correct cfi_negate_ra_state unwind directive.

Reviewed-by: Matthieu Longo <matthieu.longo@arm.com>