Fix struct pt_regs declaration on i386 and x86-64

* defs.h [I386] (i386_regs): Replace definition with declaration.
[X86_64] (x86_64_regs): Remove.
* syscall.c [X86_64] (x86_64_regs): Make static.
diff --git a/defs.h b/defs.h
index 2238373..45e3246 100644
--- a/defs.h
+++ b/defs.h
@@ -393,9 +393,7 @@
 
 #ifdef LINUX
 # if defined(I386)
-struct pt_regs i386_regs;
-# elif defined(X86_64)
-struct pt_regs x86_64_regs;
+extern struct pt_regs i386_regs;
 # endif
 #endif /* LINUX */