powerpc: rework sparse for lib/xor_vmx.c

Sparse doesn't seem to be passing -maltivec around properly, leading
to lots of errors:

.../include/altivec.h:34:2: error: Use the "-maltivec" flag to enable PowerPC AltiVec support
arch/powerpc/lib/xor_vmx.c:27:16: error: Expected ; at end of declaration
arch/powerpc/lib/xor_vmx.c:27:16: error: got signed
arch/powerpc/lib/xor_vmx.c:60:9: error: No right hand side of '*'-expression
arch/powerpc/lib/xor_vmx.c:60:9: error: Expected ; at end of statement
arch/powerpc/lib/xor_vmx.c:60:9: error: got v1_in
...
arch/powerpc/lib/xor_vmx.c:87:9: error: too many errors

Only include the altivec.h header for non-__CHECKER__ builds.
For builds with __CHECKER__, make up some stubs instead, as
suggested by Balbir. (The vector size of 16 is arbitrary.)

Suggested-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Tested-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>