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/Makefile.am b/Makefile.am
index 639430d..99d9eaf 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -151,6 +151,7 @@
 	linux/sparc64/syscallent.h	\
 	linux/sparc64/syscallent1.h	\
 	linux/sparc64/syscallent2.h	\
+	linux/subcall.h			\
 	linux/syscall.h			\
 	linux/tile/ioctlent.h.in	\
 	linux/tile/syscallent.h		\