MIPS: KVM: Fix 64-bit big endian dynamic translation

The MFC0 and MTC0 instructions in the guest which cause traps can be
replaced with 32-bit loads and stores to the commpage, however on big
endian 64-bit builds the offset needs to have 4 added so as to
load/store the least significant half of the long instead of the most
significant half.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <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>