CmmToC: enable 64-bit CallishMachOp on 32-bit targets

Normally, the unregisterised builds avoid generating 64-bit CallishMachOp in StgToCmm, so CmmToC doesn't support these. However, there do exist cases where we'd like to invoke cmmToC for other cmm inputs which may contain such CallishMachOps, and it's a rather low effort to add support for these since they only require calling into existing ghc-prim cbits.