fix atomic intrinsics implementation issues

[vtjnash]

• jltypes: add missing GC root for cmpswap_type Tuple This is called with a fieldtype, which might not even be a DataType.

• support Ptr{Union{}} and Ptr{Cvoid} better

As seen on CI run here: https://buildkite.com/julialang/julia-master/builds/24304#018858f7-adce-4928-8aad-51ad6584b439

[KristofferC]

Backporting this I get:

/home/kc/julia1.9/src/jltypes.c: In function 'ijl_apply_cmpswap_type':
./jl_internal_funcs.inc:16:31: warning: initialization of 'jl_value_t *' {aka 'struct _jl_value_t *'} from incompatible pointer type 'jl_tupletype_t *' {aka 'struct _jl_datatype_t *'} [-Wincompatible-pointer-types]
   16 | #define jl_apply_tuple_type_v ijl_apply_tuple_type_v
      |                               ^~~~~~~~~~~~~~~~~~~~~~
/home/kc/julia1.9/src/jltypes.c:1187:26: note: in expansion of macro 'jl_apply_tuple_type_v'
 1187 |     jl_value_t *tuptyp = jl_apply_tuple_type_v(params, 2);

[vtjnash]

It just needs a cast to remove the warning