powerpc: Use Linux kABI for syscall return

It changes the powerpc INTERNAL_VSYSCALL_CALL and INTERNAL_SYSCALL_NCS
to return a negative value instead of the returning the CR value in
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 on powerpc64-linux-gnu, powerpc64le-linux-gnu, and
powerpc-linux-gnu-power4.