s390: add USER_STACKTRACE support

Use the perf_callchain_user() code as blueprint to also add support for
USER_STACKTRACE. To describe how to use this cite the commit message of the
LoongArch implementation which came with commit 4d7bf939 ("LoongArch:
Add USER_STACKTRACE support"), but replace -fno-omit-frame-pointer option
with the s390 specific -mbackchain option:
====================================================================== To
get the best stacktrace output, you can compile your userspace programs
with frame pointers (at least glibc + the app you are tracing).

1, export "CC = gcc -mbackchain";
2, compile your programs with "CC";
3, use uprobe to get stacktrace output.

...
     echo 'p:malloc /usr/lib64/libc.so.6:0x0a4704 size=%r2:u64' > uprobe_events
     echo 'p:free /usr/lib64/libc.so.6:0x0a4d50 ptr=%r2:u64' >> uprobe_events
     echo 'comm == "demo"' > ./events/uprobes/malloc/filter
     echo 'comm == "demo"' > ./events/uprobes/free/filter
     echo 1 > ./options/userstacktrace
     echo 1 > ./options/sym-userobj
...
======================================================================

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