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

include/os/linux/spl/sys/user.h

@@ -0,0 +1,42 @@
+/*
+ *  Copyright (C) 2015 Cluster Inc.
+ *  Produced at ClusterHQ Inc (cf, DISCLAIMER).
+ *  Written by Richard Yao <richard.yao@clusterhq.com>.
+ *
+ *  This file is part of the SPL, Solaris Porting Layer.
+ *  For details, see <http://zfsonlinux.org/>.
+ *
+ *  The SPL is free software; you can redistribute it and/or modify it
+ *  under the terms of the GNU General Public License as published by the
+ *  Free Software Foundation; either version 2 of the License, or (at your
+ *  option) any later version.
+ *
+ *  The SPL is distributed in the hope that it will be useful, but WITHOUT
+ *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ *  for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef _SPL_USER_H
+#define	_SPL_USER_H
+
+/*
+ * We have uf_info_t for areleasef(). We implement areleasef() using a global
+ * linked list of all open file descriptors with the task structs referenced,
+ * so accessing the correct descriptor from areleasef() only requires knowing
+ * about the Linux task_struct. Since this is internal to our compatibility
+ * layer, we make it an opaque type.
+ *
+ * XXX: If the descriptor changes under us and we do not do a getf() between
+ * the change and using it, we would get an incorrect reference.
+ */
+
+struct uf_info;
+typedef struct uf_info uf_info_t;
+
+#define	P_FINFO(x) ((uf_info_t *)x)
+
+#endif /* SPL_USER_H */