s390/dumpstack: implement and use return_address()

Implement return_address() and use it instead of __builtin_return_address(n).

__builtin_return_address(n) is not guaranteed to work for n > 0,
therefore implement a private return_address() function which walks
the stack frames and returns the proper return address.

This way we get also rid of a compile warning which gcc 6.1 emits and
look like all other architectures.

Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Martin Schwidefsky <[email protected]>

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