powerpc/kvm: Fix build with older toolchains

powerpc/kvm: Fix build with older toolchains

Fix KVM build for older toolchains (found with .powerpc64-unknown-linux-gnu-gcc
(crosstool-NG-1.8.1) 4.3.2):

  AS      arch/powerpc/kvm/book3s_hv_rmhandlers.o
arch/powerpc/kvm/book3s_hv_rmhandlers.S: Assembler messages:
arch/powerpc/kvm/book3s_hv_rmhandlers.S:1388: Error: Unrecognized opcode: `popcntw'
make[1]: *** [arch/powerpc/kvm/book3s_hv_rmhandlers.o] Error 1
make: *** [_module_arch/powerpc/kvm] Error 2

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>