Issues with crt2.o and dllcrt2.o from mingw.

[crlf0710]

Those crt2.o and dllcrt2.o files have to match the gcc.exe used. So for example when i use win-gnu target in a msvc toolchain, I'll install my own copy of MINGW, and it should provide gcc.exe and those .o files, and i shouldn't use the rust-provided one.

So I suggest:

1. We package these files in "rust-mingw" component instead of current "rust-std" component in win-gnu builds.
2. Make people able to use an environment variable or something to specify where to pick these .o files in the linker command.