__xchg for sizes other than 32bit

[Imre Deak]

Hi,

recently you've added support for __cmpxchg for ARMv6+. There the assumption
is that ldrex[bh]/strex[bh] is only supported on platforms with the 32v6K
extension.

Currently the __xchg code uses these even without the extension. Should this
be fixed?

I'm looking at this since I'd need to add support for short arguments in __xchg
needed at least by the cgroups code.

Thanks,
Imre