arm64: bti: Document behaviour for dynamically linked binaries

For dynamically linked binaries the interpreter is responsible for setting
PROT_BTI on everything except itself. The dynamic linker needs to be aware
of PROT_BTI, for example in order to avoid dropping that when marking
executable pages read only after doing relocations, and doing everything
in userspace ensures that we don't get any issues due to divergences in
behaviour between the kernel and dynamic linker within a single executable.
Add a comment indicating that this is intentional to the code to help
people trying to understand what's going on.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>