selftests/ftrace: Use readelf to find entry point in uprobe test

selftests/ftrace: Use readelf to find entry point in uprobe test

The uprobe events test fails on s390, but also on x86 (Fedora 41). The
problem appears to be that there is an assumption that adding a uprobe to
the beginning of the executable mapping of /bin/sh is sufficient to trigger
a uprobe event when /bin/sh is executed.

This assumption is not necessarily true. Therefore use "readelf -h" to find
the entry point address of /bin/sh and use this address when adding the
uprobe event.

This adds a dependency to readelf which is not always installed. Therefore
add a check and exit with exit_unresolved if it is not installed.

Link: https://lore.kernel.org/r/20250220130102.2079179-1-hca@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>