kselftest/arm64: Explicitly build no BTI tests with BTI disabled

In case a distribution enables branch protection by default do as we do for
the main kernel and explicitly disable branch protection when building the
test case for having BTI disabled to ensure it doesn't get turned on by the
toolchain defaults.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220516182213.727589-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>