PR27030 bpf/uconversions.stp: other sizes user_*_error()

PR27030 bpf/uconversions.stp: other sizes user_*_error()

* tapset/bpf/uconversions.stp (__bpf_probe_read_user_error): Don't shift by 8-size.
(__signext): New function.
(__bpf_probe_read_user_error_signed): New function.
(user_*_error): New functions, all from tapset/uconversions.stp.
* tapset/uconversions.stp (user_*_error): Disable these definitions on bpf backend.
* testsuite/systemtap.bpf/bpf-target.c: Extend test to probe different size values.
* testsuite/systemtap.bpf/bpf_target_tests/user_long.stp: Ditto.