um: Add generic stub_syscall1 function

The 64bit version did not have a stub_syscall1 function yet. Add it as
it will be useful to implement a static binary for stub loading.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20240919124511.282088-2-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>