arm64: BTI: Add Kconfig entry for userspace BTI

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: Mark Brown <broonie@kernel.org>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>