powerpc: Fix reading and writing SPRs from xmon on 32-bit

When we created the instructions to read/write SPRs in xmon, we were
setting up a ppc64-style procedure descriptor and calling that, which
doesn't work in 32-bit.  For 32-bit a function pointer just points
to the instructions of the function.  This fixes it to do the right
thing for both 32-bit and 64-bit.

Signed-off-by: Paul Mackerras <paulus@samba.org>