OpenZFS restructuring - move platform specific headers

Move platform specific Linux headers under include/os/linux/.
Update the build system accordingly to detect the platform.
This lays some of the initial groundwork to supporting building
for other platforms.

As part of this change it was necessary to create both a user
and kernel space sys/simd.h header which can be included in
either context.  No functional change, the source has been
refactored and the relevant #include's updated.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Matthew Macy <mmacy@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9198

module/zcommon/zfs_fletcher.c

@@ -137,10 +137,10 @@
 #include <sys/sysmacros.h>
 #include <sys/byteorder.h>
 #include <sys/spa.h>
+#include <sys/simd.h>
 #include <sys/zio_checksum.h>
 #include <sys/zfs_context.h>
 #include <zfs_fletcher.h>
-#include <linux/simd.h>
 
 #define	FLETCHER_MIN_SIMD_SIZE	64
 
@@ -773,7 +773,7 @@ fletcher_4_init(void)
 	/*
 	 * For 5.0 and latter Linux kernels the fletcher 4 benchmarks are
 	 * run in a kernel threads.  This is needed to take advantage of the
-	 * SIMD functionality, see include/linux/simd_x86.h for details.
+	 * SIMD functionality, see linux/simd_x86.h for details.
 	 */
 	taskqid_t id = taskq_dispatch(system_taskq, fletcher_4_benchmark,
 	    NULL, TQ_SLEEP);

module/zcommon/zfs_fletcher_aarch64_neon.c

@@ -43,7 +43,7 @@
 
 #if defined(__aarch64__)
 
-#include <linux/simd_aarch64.h>
+#include <sys/simd.h>
 #include <sys/spa_checksum.h>
 #include <sys/strings.h>
 #include <zfs_fletcher.h>

module/zcommon/zfs_fletcher_avx512.c

@@ -24,11 +24,11 @@
 
 #if defined(__x86_64) && defined(HAVE_AVX512F)
 
-#include <linux/simd_x86.h>
 #include <sys/byteorder.h>
 #include <sys/frame.h>
 #include <sys/spa_checksum.h>
 #include <sys/strings.h>
+#include <sys/simd.h>
 #include <zfs_fletcher.h>
 
 #define	__asm __asm__ __volatile__

module/zcommon/zfs_fletcher_intel.c

@@ -42,8 +42,8 @@
 
 #if defined(HAVE_AVX) && defined(HAVE_AVX2)
 
-#include <linux/simd_x86.h>
 #include <sys/spa_checksum.h>
+#include <sys/simd.h>
 #include <sys/strings.h>
 #include <zfs_fletcher.h>
 

module/zcommon/zfs_fletcher_sse.c

@@ -43,7 +43,7 @@
 
 #if defined(HAVE_SSE2)
 
-#include <linux/simd_x86.h>
+#include <sys/simd.h>
 #include <sys/spa_checksum.h>
 #include <sys/byteorder.h>
 #include <sys/strings.h>