Now that the code for userspace BTI support is in the kernel add the
Kconfig entry so that it can be built and used.

[Split out of "arm64: Basic Branch Target Identification support"
    -- broonie]
Signed-off-by: NMark Brown <broonie@kernel.org>
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Reviewed-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>