I've exported these functions from GHC.Exts.  They are still
implemented using the FFI underneath, though.

To avoid conditional exports, on a 64-bit build:

  uncheckedShiftL64# = uncheckShiftL#

(etc.) which has a different type than the 32-bit version of
uncheckedShiftL64#, but at least GHC.Exts exports the same names.