KVM: arm64: Add templates for BHB mitigation sequences

KVM writes the Spectre-v2 mitigation template at the beginning of each
vector when a CPU requires a specific sequence to run.

Because the template is copied, it can not be modified by the alternatives
at runtime.

Add templates for calling ARCH_WORKAROUND_3 and one for each value of K
in the brancy-loop. Add dummy functions to identify these mitigations,
and a copy of install_bp_hardening_cb() that is able to install them.

Signed-off-by: James Morse <james.morse@arm.com>