[S390] remove warnings with functions ending in BUG

Functions which end in a BUG() statement and skip the return statement
cause compile warnings on s390, e.g.:

mm/bootmem.c: In function 'mark_bootmem':
mm/bootmem.c:321: warning: control reaches end of non-void function

To avoid the warning add an endless loop to the BUG() macro.
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/bug.h

@@ -47,7 +47,10 @@
 
 #endif /* CONFIG_DEBUG_BUGVERBOSE */
 
-#define BUG()	__EMIT_BUG(0)
+#define BUG() do {					\
+	__EMIT_BUG(0);					\
+	for (;;);					\
+} while (0)
 
 #define WARN_ON(x) ({					\
 	int __ret_warn_on = !!(x);			\