[feature] Add support of custom arch

[sr-tream]

Since 2018 DUB may pass -mtriple targets to ldc2 from -a/--arch= argument, like dub build -ax86_64-windows-msvc --complier=ldc2.

It would be great if I can use x86_64-windows-msvc for building from vscode instead of usage terminal

[WebFreak001]

already supported in current nightly serve-d

[WebFreak001]

pre-release available, next code-d release will also expose this better.