s390/test_unwind: fix and extend kprobes test

Running kprobe test on a kernel built with clang 14 didn't actually
trigger pgm_pre_handler() and no unwinder code was called. Even though
do_report_trap() is a global symbol, clang inlined it in several local
functions including illegal_op() handler, so that kprobbing a global
symbol didn't have a desired effect.

To achieve the same test result (unwinding from a program check
handler) introduce a local function and probe an instruction in the
middle, so that kprobe doesn't take KPROBE_ON_FTRACE path.

While at it, add another test for KPROBE_ON_FTRACE.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>