Refactor librustc_back to support arbitrary linkers

[retep998]

Currently librustc_back is hardcoded to use a certain method of linking which makes it extremely inflexible for supporting other linkers such as link.exe. It needs to be refactored somehow such that a flexible target spec can specify what linker implementation to use.

[huonw]

Flexible target specs are loaded at runtime into the compiler, and don't allow implementing a trait for custom behaviour.

(It seems it may not be better to prescribe an implementation strategy.)

[retep998]

This would be fixed by #25350 which does in fact use a trait for the linker as I originally suggested. ( @huonw 😛 )