Enhancement on `export` symbol

[kinke]

With LDC, you can use `-fvisibility={public,hidden}` to control 
the default visibility (for symbols without explicit `export` 
visibility / `@hidden` UDA). It defaults to `public` when 
compiling & linking a DLL in a single step via `-shared`; when 
compiling the DLL separately without linking, you need to specify 
it yourself.

Recent DMD has that CLI option too, `-visibility` (no leading 
`f`).

For (a lot of) context, see 
https://forum.dlang.org/post/dobouzmhwabquswguunk@forum.dlang.org.