linux-user: Fix invalid TARGET_ABI_BITS usage on ppc hosts

When trying to evaluate the size of the _host_ type size for olddev_t,
we need to expose the host's pointer size, not the guest pointer size.

This usage got introduced accidently in commit b754e4fc.

Fix things by not using TARGET_.*, but rather use host sizeof()
information, which gives us the correct size.
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAlexander Graf <agraf@suse.de>

thunk.h

@@ -113,7 +113,7 @@ static inline int thunk_type_size(const argtype *type_ptr, int is_host)
       defined(HOST_PARISC) || defined(HOST_SPARC64)
             return 4;
 #elif defined(HOST_PPC)
-            return TARGET_ABI_BITS / 8;
+            return sizeof(void *);
 #else
             return 2;
 #endif