aarch64: enable PAC and BTI

Enable Pointer Authentication Codes (PAC) and Branch Target
Identification (BTI) support for ARM 64 targets.

PAC works by signing the LR with either an A key or B key and verifying
the return address. There are quite a few instructions capable of doing
this, however, the Linux ARM ABI is to use hint compatible instructions
that can be safely NOP'd on older hardware and can be assembled and
linked with older binutils. This limits the instruction set to paciasp,
pacibsp, autiasp and autibsp. Instructions prefixed with pac are for
signing and instructions prefixed with aut are for signing. Both
instructions are then followed with an a or b to indicate which signing
key they are using. The keys can be controlled using
-mbranch-protection=pac-ret for the A key and
-mbranch-protection=pac-ret+b-key for the B key.

BTI works by marking all call and jump positions with bti c and bti
j instructions. If execution control transfers to an instruction other
than a BTI instruction, the execution is killed via SIGILL. Note that
to remove one instruction, the aforementioned pac instructions will
also work as a BTI landing pad for bti c usages.

For BTI to work, all object files linked for a unit of execution,
whether an executable or a library must have the GNU Notes section of
the ELF file marked to indicate BTI support. This is so loader/linkers
can apply the proper permission bits (PROT_BRI) on the memory region.

PAC can also be annotated in the GNU ELF notes section, but it's not
required for enablement, as interleaved PAC and non-pac code works as
expected since it's the callee that performs all the checking.

Signed-off-by: Bill Roberts <[email protected]>