Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC: r241374

Add an unified macro to deny ability from the compiler to reorder
instruction loads/stores at its will.
The macro __compiler_membar() is currently supported for both gcc and
clang, but kernel compilation will fail otherwise.

Reviewed by: bde, kib
Discussed with: dim, theraven

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Add memmove() to the kernel, making the kernel compile with Clang.

When copying big structures, LLVM generates calls to memmove(), because
it may not be able to figure out whether structures overlap. This caused
linker errors to occur. memmove() is now implemented using bcopy().
Ideally it would be the other way around, but that can be solved in the
future. On ARM we don't do add anything, because it already has
memmove().

Discussed on: arch@
Reviewed by: rdivacky

Set magic fbsd:nokeywords property that allows files to bypass
keyword expansion. (file-specific replacement for CVSROOT/exclude)

Sync XFS for FreeBSD tree with newer changes from SGI XFS for Linux tree.
Improve support for writing to XFS partitions.

Work done by: Russell Cattelan <cattelan at xfs dot org>

Initial import of read-only support for SGI's XFS filesystem.

Contributed by: XFS for FreeBSD project