um: mdd support for 64 bit atomic operations

This adds support for 64 bit atomic operations on 32 bit UML systems.  XFS
needs them since 2.6.38.

  $ make ARCH=um SUBARCH=i386
  ...
    LD      .tmp_vmlinux1
  fs/built-in.o: In function `xlog_regrant_reserve_log_space':
  xfs_log.c:(.text+0xd8584): undefined reference to `atomic64_read_386'
  xfs_log.c:(.text+0xd85ac): undefined reference to `cmpxchg8b_emu'
  ...

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=32812



Reported-by: Martin Walch <walch.martin@web.de>
Tested-by: Martin Walch <walch.martin@web.de>
Cc: Martin Walch <walch.martin@web.de>
Cc: <stable@kernel.org> [2.6.38.x 084189a8: um: disable CONFIG_CMPXCHG_LOCAL]
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>