syscall.c: split arch specific code into separate arch files

Split code that use arch-specific registers to separate arch files.

* syscall.c: Move definitions of variables containing fetched registers
to linux/*/arch_regs.c files.
[HAVE_GETRVAL2] (getrval2): Move arch-specific code
to linux/*/arch_getrval2.c, include "arch_getrval2.c".
(print_pc): Move arch-specific code to linux/*/print_pc.c files,
include "print_pc.c".
[X86_64] (x86_64_getregs_old): Rename to getregs_old, move to
linux/x86_64/getregs_old.c, include "getregs_old.c".
[POWERPC] (powerpc_getregs_old): Rename to getregs_old, move to
linux/powerpc/getregs_old.c, include "getregs_old.c".
(get_regs) [X86_64, POWERPC]: Update callers.
(get_scno): Move arch-specific code to linux/*/get_scno.c,
include "get_scno.c".
(get_syscall_args): Move arch-specific code
to linux/*/get_syscall_args.c, include "get_syscall_args.c".
(get_error): Move arch-specific code to linux/*/get_error.c,
include "get_error.c".
(get_syscall_result): Move arch-specific code
to linux/*/get_syscall_result.c, include "get_syscall_result.c".
* Makefile.am (EXTRA_DIST): Add new linux/*/*.c files.
diff --git a/linux/m68k/arch_regs.c b/linux/m68k/arch_regs.c
new file mode 100644
index 0000000..5d41825
--- /dev/null
+++ b/linux/m68k/arch_regs.c
@@ -0,0 +1 @@
+static long m68k_d0;