arm64: Make the Meltdown mitigation state available

Our Meltdown mitigation state isn't exposed outside of cpufeature.c,
contrary to the rest of the Spectre mitigation state. As we are going
to use it in KVM, expose a arm64_get_meltdown_state() helper which
returns the same possible values as arm64_get_spectre_v?_state().

Signed-off-by: Marc Zyngier <maz@kernel.org>