[REG 2.066] Fix 13399 "va_arg is nothrow yet may throw "

Fixes https://issues.dlang.org/show_bug.cgi?id=13399

comment from bugzilla:

> Have encountered same issue when trying to build vibe.d newsgroup server. Looking at the content of the `stdarg` module it doesn't feel like module-wide `nothrow` is applicable there at all, there are just too many things that can throw indirectly. In my opinion this needs to be reverted and `nothrow` added on per-function basis