target/ppc: added ifdefs around TCG-only code

target/ppc: added ifdefs around TCG-only code

excp_helper.c, mmu-hash64.c and mmu_helper.c have some function
declarations that are TCG-only, and couldn't be easily moved to a
TCG only file, so ifdefs were added around them.

We also needed ifdefs around some header files because helper-proto.h
includes trace/generated-helpers.h, which is never created when building
without TCG, and cpu_ldst.h includes tcg/tcg.h, whose containing folder
is not included as a -iquote. As future cleanup, we could change the
part of the configuration script to add those.

cpu_init.c also had a callback definition that is TCG only and could be
removed as part of a future cleanup (all the dump_statistics part is
almost never used and will become obsolete as we transition to using
decodetree).

Signed-off-by: Bruno Larsen (billionai) <bruno.larsen@eldorado.org.br>
Message-Id: <20210525115355.8254-3-bruno.larsen@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>