Normalize the declaration of i386_read_exec variable.

It is currently re-declared in sys/sysent.h which is a wrong place for
MD variable.  Which causes redeclaration error with gcc when
sys/sysent.h and machine/md_var.h are included both.

Remove it from sys/sysent.h and instead include machine/md_var.h when
needed, under #ifdef for both i386 and amd64.

Reported and tested by:	bde
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/compat/freebsd32/freebsd32_misc.c

@@ -106,6 +106,9 @@ __FBSDID("$FreeBSD$");
 
 #include <machine/cpu.h>
 #include <machine/elf.h>
+#ifdef __amd64__
+#include <machine/md_var.h>
+#endif
 
 #include <security/audit/audit.h>
 

sys/sys/sysent.h

@@ -163,10 +163,6 @@ extern struct sysentvec aout_sysvec;
 extern struct sysent sysent[];
 extern const char *syscallnames[];
 
-#if defined(__amd64__)
-extern int i386_read_exec;
-#endif
-
 #define	NO_SYSCALL (-1)
 
 struct module;

sys/vm/vm_mmap.c

@@ -74,6 +74,9 @@ __FBSDID("$FreeBSD$");
 #include <sys/syscallsubr.h>
 #include <sys/sysent.h>
 #include <sys/vmmeter.h>
+#if defined(__amd64__) || defined(__i386__) /* for i386_read_exec */
+#include <machine/md_var.h>
+#endif
 
 #include <security/audit/audit.h>
 #include <security/mac/mac_framework.h>

sys/vm/vm_unix.c

@@ -55,6 +55,9 @@ __FBSDID("$FreeBSD$");
 #include <sys/sysent.h>
 #include <sys/sysproto.h>
 #include <sys/systm.h>
+#if defined(__amd64__) || defined(__i386__) /* for i386_read_exec */
+#include <machine/md_var.h>
+#endif
 
 #include <vm/vm.h>
 #include <vm/vm_param.h>