[OpenMP] Simplify generic microtask dispatch (#214004)

Replace the generic microtask dispatcher's handwritten function-pointer
types and calls with a variadic-template helper and a macro for each
supported argument count.

The dispatcher still casts each microtask to the exact fixed-arity
signature required by platforms such as WebAssembly. This is intended as
a refactor only: the existing limit of 15 microtask arguments and the
diagnostic for larger argument counts remain unchanged.

This reduces the repetitive code and makes future changes to the
supported argument range easier to review.

Split out from #211071

GitOrigin-RevId: af14456a2bac55026b2d57cbf14b3905c452b8a4