fix: signature lowering for strings and lists on modules with 64bit memory

[christoph-dfinity]

Fixes signature lowering for strings and lists on modules with 64bit memory.

Before this change the newly added tests fail with a signature mismatch, because the lowering unconditionally produced i32 pointers and lengths. On modules with 64bit memories these need to be i64 instead, based on my reading of the Canonical ABI document

Before the fix the tests fail with:

---- "tests/cli/component-model/memory64/string.wast" ----
failed test "tests/cli/component-model/memory64/string.wast"
...
       Caused by:
           0: failed testing wasm binary produced by `wast`
           1: wasm isn't valid
           2: lowered parameter types `[I32, I32]` do not match parameter types `[I64, I64]` of core function 0 (at offset 0x8f)

/cc @michael-weigelt