[S390] Fix a lot of sparse warnings.
Most noteable part of this commit is the new local header file entry.h which contains all the function declarations of functions that get only called from asm code or are arch internal. That way we can avoid extern declarations in C files. This is more or less the same that was done for sparc64. Signed-off-by: Martin Schwidefsky <firstname.lastname@example.org> Signed-off-by: Heiko Carstens <email@example.com>