KVM: x86: compile out TDP MMU on 32-bit systems

The TDP MMU assumes that it can do atomic accesses to 64-bit PTEs.
Rather than just disabling it, compile it out completely so that it
is possible to use for example 64-bit xchg.

To limit the number of stubs, wrap all accesses to tdp_mmu_enabled
or tdp_mmu_page with a function.  Calls to all other functions in
tdp_mmu.c are eliminated and do not even reach the linker.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Tested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>