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>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -1263,7 +1263,7 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_206)
 	addi	r6,r5,VCORE_NAPPING_THREADS
 31:	lwarx	r4,0,r6
 	or	r4,r4,r0
-	popcntw	r7,r4
+	PPC_POPCNTW(r7,r4)
 	cmpw	r7,r8
 	bge	2f
 	stwcx.	r4,0,r6