[TBI](https://en.wikichip.org/wiki/arm/tbi) is a very useful feature of ARMv8 that allows the top byte of pointers to be used as a tag. This is useful for various JIT compilers, [HWASAN](https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html) etc. This has been enabled on Linux since 2013, but not on FreeBSD yet.
[Linux clears the bits on EL0 data abort](https://github.com/torvalds/linux/commit/d50240a5f6ceaf690a77b0fccb17be51cfa151c2#diff-5852c4f382c23511d037a653b271b3fd), so I've done the same (??) here — not sure about that actually.