Compiling drm-kmod on !X86 does not include asm/smp.h which includes
preempt.h on FreeBSD.  In order to compile drm-kmod on other
architectures add the secondary #includes for preempt.h to
spinlock.h and hardirq.h (which now also gets included from highmem.h)
to connect the #include chain.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
PR:		279864
Reviewed by:	jhibbits, emaste
Differential Revision: https://reviews.freebsd.org/D55974

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

It currently does the same as `synchronize_irq()`.

Reviewed by:	manu
Approved by:	manu
Differential Revision:	https://reviews.freebsd.org/D38079

MFC after:	1 week
Reviewed by:	bz, emaste, hselasky, manu
Differential Revision:	https://reviews.freebsd.org/D33562

Linux does the same, this avoids ifdef or extra includes in ported drivers.

Reviewed by:	emaste, hselasky
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25702

- Move all files related to the LinuxKPI into sys/compat/linuxkpi and
  its subfolders.
- Update sys/conf/files and some Makefiles to use new file locations.
- Added description of COMPAT_LINUXKPI to sys/conf/NOTES which in turn
  adds the LinuxKPI to all LINT builds.
- The LinuxKPI can be added to the kernel by setting the
  COMPAT_LINUXKPI option. The OFED kernel option no longer builds the
  LinuxKPI into the kernel. This was done to keep the build rules for
  the LinuxKPI in sys/conf/files simple.
- Extend the LinuxKPI module to include support for USB by moving the
  Linux USB compat from usb.ko to linuxkpi.ko.
- Bump the FreeBSD_version.
- A universe kernel build has been done.

Reviewed by:	np @ (cxgb and cxgbe related changes only)
Sponsored by:	Mellanox Technologies