[BOLT] Gadget scanner: optionally assume auth traps on failure
On AArch64 it is possible for an auth instruction to either return an
invalid address value on failure (without FEAT_FPAC) or generate an
error (with FEAT_FPAC). It thus may be possible to never emit explicit
pointer checks, if the target CPU is known to support FEAT_FPAC.
This commit implements an --auth-traps-on-failure command line option,
which essentially makes "safe-to-dereference" and "trusted" register
properties identical and disables scanning for authentication oracles
completely.