stage2 can't cross compile mingw

[Vexu]

To reproduce on Linux:

$ touch empty.c
$ echo "pub fn main() void {}" > main.zig
$ zig build-exe a.zig a.c -target x86_64-windows -fstage1
$ zig build-exe main.zig empty.c -target x86_64-windows
error(mingw): clang exited with code 1 and stderr: error: unknown target triple 'native-unknown-linux-gnu', please use -triple or -arch

error(mingw): clang exited with code 1 and stderr: error: unknown target triple 'native-unknown-linux-gnu', please use -triple or -arch

error: unable to generate DLL import .lib file: ClangPreprocessorFailed