Move structs into unnamed namespaces

[StephanTLavavej]

When the STL's separately compiled code needs helper structs, we often define them in unnamed namespaces. This is simpler to understand (making it clear that the helpers are specific to one translation unit), and cleaner for static linking (unnamed namespaces now grant internal linkage).

I noticed a few things that weren't following this convention - the recently-added XFG trampoline, and the system_error tables. This PR moves the structs (and the tables) into top-level unnamed namespaces. It additionally removes static from the namespace-scope constexpr tables, as that's super redundant.

(We wouldn't need to use _STD for errc and _Execute_once_fp_t if these were unnamed namespaces within std. However, we appear to have generally avoided that, with the exception of some Concurrency::details::unnamed-namespace code.)

This does not attempt to move all such structs into unnamed namespaces; there some helper structs for iostreams that I consider way too scary to change.