KVM: MIPS: Fix build errors for 32bit kernel

Commit dc6d95b1 ("KVM: MIPS: Add more MMIO load/store
instructions emulation") introduced some 64bit load/store instructions
emulation which are unavailable on 32bit platform, and it causes build
errors:

arch/mips/kvm/emulate.c: In function 'kvm_mips_emulate_store':
arch/mips/kvm/emulate.c:1734:6: error: right shift count >= width of type [-Werror]
      ((vcpu->arch.gprs[rt] >> 56) & 0xff);
      ^
arch/mips/kvm/emulate.c:1738:6: error: right shift count >= width of type [-Werror]
      ((vcpu->arch.gprs[rt] >> 48) & 0xffff);
      ^
arch/mips/kvm/emulate.c:1742:6: error: right shift count >= width of type [-Werror]
      ((vcpu->arch.gprs[rt] >> 40) & 0xffffff);
      ^
arch/mips/kvm/emulate.c:1746:6: error: right shift count >= width of type [-Werror]
      ((vcpu->arch.gprs[rt] >> 32) & 0xffffffff);
      ^
arch/mips/kvm/emulate.c:1796:6: error: left shift count >= width of type [-Werror]
      (vcpu->arch.gprs[rt] << 32);
      ^
arch/mips/kvm/emulate.c:1800:6: error: left shift count >= width of type [-Werror]
      (vcpu->arch.gprs[rt] << 40);
      ^
arch/mips/kvm/emulate.c:1804:6: error: left shift count >= width of type [-Werror]
      (vcpu->arch.gprs[rt] << 48);
      ^
arch/mips/kvm/emulate.c:1808:6: error: left shift count >= width of type [-Werror]
      (vcpu->arch.gprs[rt] << 56);
      ^
cc1: all warnings being treated as errors
make[3]: *** [arch/mips/kvm/emulate.o] Error 1

So, use #if defined(CONFIG_64BIT) && defined(CONFIG_KVM_MIPS_VZ) to
guard the 64bit load/store instructions emulation.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: dc6d95b1 ("KVM: MIPS: Add more MMIO load/store instructions emulation")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Message-Id: <1594365797-536-1-git-send-email-chenhc@lemote.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>