Allocgate missing runtime-known alignment functions

[silversquirl]

To quote @andrewrk on #10112:

[...] I agree that this use case will need to continue to be supported - runtime-known alignment.

However, the new allocator interface does not feature any high-level functions to support this. Ideally there would be a reallocAdvancedWithRetAddrRuntimeAlignment (though that name is rather unwieldy).

[andrewrk]

Hmm, looks like #9394 regressed with those changes.