Move subcall decoding configuration out of the common code

* Makefile.am (EXTRA_DIST): Add linux/subcall.h.
* linux/syscall.h (SYS_socket_subcall): Remove.
(SYS_socket_nsubcalls, SYS_ipc_subcall, SYS_ipc_nsubcalls): Move to ...
* linux/subcall.h: ... new file.
* linux/arm/syscallent.h: Define SYS_socket_subcall, include subcall.h.
* linux/i386/syscallent.h: Likewise.
* linux/m68k/syscallent.h: Likewise.
* linux/powerpc/syscallent.h: Likewise.
* linux/s390/syscallent.h: Likewise.
* linux/s390x/syscallent.h: Likewise.
* linux/sh/syscallent.h: Likewise.
* linux/sh64/syscallent.h: Likewise.
* linux/sparc/syscallent.h: Likewise.

Suggested by Mike Frysinger and Chris Metcalf.
diff --git a/linux/sh/syscallent.h b/linux/sh/syscallent.h
index 577a4f6..af06962 100644
--- a/linux/sh/syscallent.h
+++ b/linux/sh/syscallent.h
@@ -430,10 +430,8 @@
 	{ 5,	0,	NULL,			NULL		}, /* 397 */
 	{ 5,	0,	NULL,			NULL		}, /* 398 */
 	{ 5,	0,	NULL,			NULL		}, /* 399 */
-
-#if SYS_socket_subcall != 400
- #error fix me
-#endif
+#define SYS_socket_subcall	400
+#include "subcall.h"
 	{ 6,	0,	printargs,		"socket_subcall"}, /* 400 */
 	{ 3,	TN,	sys_socket,		"socket"	}, /* 401 */
 	{ 3,	TN,	sys_bind,		"bind"		}, /* 402 */