target-ppc: fix tcg fatal error on i386 host

It looks like the i386 runs out of registers for allocation due
to too many global registers allocated by the ppc target.

Here is a quick and dirty fix that seems to solve the problem.
This should be considered as temporary.

Signed-off-by: Laurent Desnogues <laurent.desnogues@gmail.com
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5648 c046a42c-6fe2-441c-8c8c-71466251a162