Darwin: Check for x86_64 only on i386

The sysctl variable if we're 64-bit capable only exists on i386. So we should only check it if we're on i386.

This suppresses a warning on PowerPC spotted by Andreas Faerber.

Signed-off-by: Alexander Graf <alex@csgraf.de>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6640 c046a42c-6fe2-441c-8c8c-71466251a162
