Fix string support in CUDA target - #7052

This fixes the following failure, which occurred because unicode typing and
lowering was not registered with the CUDA target:

```
======================================================================
ERROR: test_string (numba.cuda.tests.cudapy.test_print.TestPrint)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\work\numbadev\numba\numba\core\base.py", line 710, in cast
    impl = self._casts.find((fromty, toty))
  File "C:\work\numbadev\numba\numba\core\base.py", line 48, in find
    out = self._find(sig)
  File "C:\work\numbadev\numba\numba\core\base.py", line 57, in _find
    raise NotImplementedError(self, sig)
NotImplementedError: (<numba.core.base.OverloadSelector object at 0x000001F7C7756160>, (Literal[str](hop!), unicode_type))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\work\numbadev\numba\numba\core\errors.py", line 744, in new_error_context
    yield
  File "C:\work\numbadev\numba\numba\core\lowering.py", line 230, in lower_block
    self.lower_inst(inst)
  File "C:\work\numbadev\numba\numba\core\lowering.py", line 374, in lower_inst
    self.lower_print(inst)
  File "C:\work\numbadev\numba\numba\core\lowering.py", line 729, in lower_print
    argvals = self.fold_call_args(fnty, sig, pos_args, inst.vararg, {})
  File "C:\work\numbadev\numba\numba\core\lowering.py", line 680, in fold_call_args
    argvals = [self._cast_var(var, sigty)
  File "C:\work\numbadev\numba\numba\core\lowering.py", line 680, in <listcomp>
    argvals = [self._cast_var(var, sigty)
  File "C:\work\numbadev\numba\numba\core\lowering.py", line 663, in _cast_var
    return self.context.cast(self.builder, val, varty, ty)
  File "C:\work\numbadev\numba\numba\core\base.py", line 713, in cast
    raise NotImplementedError(
NotImplementedError: Cannot cast Literal[str](hop!) to unicode_type: %".41" = load i8*, i8** %"$const16.6"
```