[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 <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
diff --git a/arch/s390/kernel/debug.c b/arch/s390/kernel/debug.c
index 95a46bc..1e7d4ac 100644
--- a/arch/s390/kernel/debug.c
+++ b/arch/s390/kernel/debug.c
@@ -157,7 +157,7 @@
 };
 
 /* used by dump analysis tools to determine version of debug feature */
-unsigned int debug_feature_version = __DEBUG_FEATURE_VERSION;
+static unsigned int __used debug_feature_version = __DEBUG_FEATURE_VERSION;
 
 /* static globals */