aarch64: group software protection options under a same struct.

- declare a new struc aarch_protection_opts to store all the
  configuration options related to software protections (i.e. bti-plt,
  pac-plt, bti-report level).
- add a new option "-z bti-report" to configure the log level of reported
  issues when BTI PLT is forced.
- encapsulate the BTI report inside _bfd_aarch64_elf_check_bti_report.