This kernel feature is required for enabling BPF_KPROBE_OVERRIDE.

Define override_function_with_return() and regs_set_return_value()
functions, and fix compile errors in syscall_wrapper.h.
Signed-off-by: NIlya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>