linux64: Fix the build on arm64 with bti checking

When we enable checking for BTI on arm64 we need to include an ELF
note in all object files linked into a module.

As using objcopy from a binary to an ELF object file doesn't add the
note switch to using .incbin from an assembly file. This allows us to
add the needed note without affecting the included object.

Reviewed by: imp, kib, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45468