Add back in the support for 64-bit PPC MacOSX hosts that was
broken in the recent merge of the 32-bit and 64-bit TCG backends.

Reported-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Tested-by: Andreas Färber <andreas.faerber@web.de>