powerpc: Fix compilation of emulate_step()

Commit be96f633 ("powerpc: Split out instruction analysis
part of emulate_step()") added some calls to do_fp_load()
and do_fp_store(), which fail to compile on configs with
CONFIG_PPC_FPU=n and CONFIG_PPC_EMULATE_SSTEP=y.  This fixes
the compile by adding #ifdef CONFIG_PPC_FPU around the code
that calls these functions.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>