mk: Add a BTI-report linker feature

Add support for specifying how to report the missing Branch Target
Identification (BTI) linker feature on AArch64.

For:

Kernel: bti-report on when the linker supports it
Userspace: bti-report on when the linker supports it and

BTI_REPORT_ERROR is defined

Fixes: 43e8849bc294 ("conf: Enable BTI checking in the arm64 kernel")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1393