FreeBSD: Sort out kernel FPU headers for 12.1-REL

We were missing an include for kernel FPU functions, breaking the build
on FreeBSD 12.1-RELEASE.  This was apparently being pulled in from
elsewhere on stable/12 and head.

Sorted the other includes in these files while here.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11005

include/os/freebsd/spl/sys/simd_x86.h

@@ -26,15 +26,12 @@
  * $FreeBSD$
  */
 
-#include <sys/cdefs.h>
 #include <sys/types.h>
-#include <sys/systm.h>
+#include <sys/cdefs.h>
 #include <sys/proc.h>
-#ifdef __i386__
-#include <x86/fpu.h>
-#else
-#include <machine/fpu.h>
-#endif
+#include <sys/systm.h>
+
+#include <machine/pcb.h>
 #include <x86/x86_var.h>
 #include <x86/specialreg.h>