MIPS: KVM: Support r6 compact branch emulation

Add support in KVM for emulation of instructions in the forbidden slot
of MIPSr6 compact branches. If we hit an exception on the forbidden
slot, then the branch must not have been taken, which makes calculation
of the resume PC trivial.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim KrÄmář <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>