target-ppc: fixes for gen_op_neg()

- Rename to gen_op_arith_neg for consistency with other functions.
- Correctly free TCG temp variable.
- Fix the return value in 64-bit mode in case of overflow.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5659 c046a42c-6fe2-441c-8c8c-71466251a162
