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 <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/ftrace.h

@@ -12,7 +12,9 @@
 
 #ifndef __ASSEMBLY__
 
-#define ftrace_return_address(n) __builtin_return_address(n)
+unsigned long return_address(int depth);
+
+#define ftrace_return_address(n) return_address(n)
 
 void _mcount(void);
 void ftrace_caller(void);