powerpc: Add macros for the ibm_architecture_vec[] lengths

The encoding of the lengths in the ibm_architecture_vec array is
"interesting" to say the least. It's non-obvious how the number of bytes
we provide relates to the length value.

In fact we already got it wrong once, see 11e9ed43 "Fix up
ibm_architecture_vec definition".

So add some macros to make it (hopefully) clearer. These at least have
the property that the integer present in the code is equal to the number
of bytes that follows it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>