linux(4): Cleanup includes under x86/linux

Cleanup unneeded includes, sort the rest according to style(9).
No functional changes.

MFC after:		2 weeks

sys/x86/linux/linux_dummy_x86.c

@@ -30,9 +30,8 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <sys/kernel.h>
-#include <sys/sdt.h>
-#include <sys/systm.h>
 #include <sys/proc.h>
+#include <sys/sdt.h>
 
 #ifdef COMPAT_LINUX32
 #include <machine/../linux32/linux.h>