um: Use RWSEM_GENERIC_SPINLOCK on x86

Commit d12337 (rwsem: Remove redundant asmregparm annotation)
broke rwsem on UML.

As we cannot compile UML with -mregparm=3 and keeping asmregparm only
for UML is inadequate the easiest solution is using RWSEM_GENERIC_SPINLOCK.

Thanks to Thomas Gleixner for the idea.

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Tested-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: user-mode-linux-devel@lists.sourceforge.net
Cc: <stable@kernel.org> # .39.x
Link: http://lkml.kernel.org/r/%3C1306183893-26655-1-git-send-email-richard%40nod.at%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>