s390: avoid __builtin_return_address(n) on clang

llvm on s390 has problems with __builtin_return_address(n), with n>0,
this results in a somewhat cryptic error message:

fatal error: error in backend: Unsupported stack frame traversal count

To work around it, use the direct return address directly. This
is probably not ideal here, but gets things to compile and should
only lead to inferior reporting, not to misbehavior of the generated
code.

Link: https://bugs.llvm.org/show_bug.cgi?id=41424
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>