ARC: [kbuild] Allow platforms to disable LLSC for !SMP as well

Currently ARC_HAS_LLSC can be influenced by platform for SMP only using
ARC_HAS_COH_LLSC. For !SMP it defaults to "y".

It turns out that some customers can't support it all, even in UP.
So we change the semantics, and use a negative dependency ARC_CANT_LLSC.
Any platform (independent of SMP or !SMP) can select it to disable LLSC.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>