arm64: Support creating a BTI & PAC note

To detect when an object file is build with the Branch Target
Identification (BTI) and Pointer Authentication Code (PAC) extensions
there is an elf note the compiler will insert. It will only do so from
a high level language, e.g. C or C++.

To get the not in assembly add the GNU_PROPERTY_AARCH64_FEATURE_1_NOTE
macro that can be used to create it, and the
GNU_PROPERTY_AARCH64_FEATURE_1_VAL macro to insert the correct value
based on which combination of BTI and PAC are enabled.

Reviewed by: markj (earlier version), emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42225