Do not merge:
- the pause hint is still in the proposal stage (https://github.com/riscv/riscv-isa-manual/pull/398)
- the syntax for representing empty sets in fence operands is not yet finalized (likely to be either 0, as in this patch, or -)
Add support for the 'pause' hint instruction as an alias for
'fence w, 0'. To do this allow the 'fence' operands pred and succ
to be set to 0 (the empty set). This will also allow future hints
to be encoded as 'fence 0, <x>' and 'fence <x>, 0'.