nios2: Use Linux kABI for syscall return

It changes the nios INTERNAL_SYSCALL_RAW macro to return a negative
value instead of the 'r2' register value on the 'err' macro argument.

The macro INTERNAL_SYSCALL_DECL is no longer required, and the
INTERNAL_SYSCALL_ERROR_P macro follows the other Linux kABIs.

Checked with a build against nios2-linux-gnu.