[openmp][wasm] Fix microtask type mismatch (#84355)

When OpenMP is compiled for WebAssembly (see #71297), it invokes a
microtask via a `switch` statement that dispatches to the `void *`
microtask pointer with spelled-out arguments (not varargs). As #83329
points out, however, this can result in a type mismatch when the
indirect call is executed by WebAssembly; WebAssembly expects the called
pointer to have the precise type of the call site. This change fixes the
issue by bringing back the approach in [D142593] of type-casting all the
`switch` arms to the precise type. This fixes #83329.

[D142593]: https://reviews.llvm.org/D142593