KVM: x86: Stub out is_tdp_mmu_root on 32-bit hosts

mainline inclusion
from mainline-v5.14-rc1
commit c62efff2
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c62efff28bb5eb60d60415a0dd0c864c64be0671



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If is_tdp_mmu_root is not inlined, the elimination of TDP MMU calls as dead
code might not work out.  To avoid this, explicitly declare the stubbed
is_tdp_mmu_root on 32-bit hosts.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>