Fix returning strings from variadic functions.

Similar to the SM fix @ https://github.com/alliedmodders/sourcemod/pull/123. Most of the patch is just adding helpers to generate the assembly text. One difference is that AMX Mod X counts arguments in bytes, whereas SourceMod counts in cells (that is, it does not have the `params[0] / sizeof(cell)` pattern). So instead of `idxaddr` to add the base and count, it just uses `add`.