rustc -C link-args=-static-libgcc does not work on Windows

[abonander]

When building on Windows, libgcc is dynamically linked by default. In the GCC Linker Options, it says that this is necessary for cross-library exception handling, but for programs that don't need that, this adds an unnecessary dynamic linking to the libgcc DLL, which would have to be distributed with the built executable since it's not available by default on Windows.

Passing -C link-args=-static-libgcc should make libgcc statically linked, but it doesn't seem to override the -shared-libgcc flag. While this suggests a bug or undocumented behavior with the GCC linker, rustc should be aware that the user wants to statically link libgcc and not add the -shared-libgcc linker flag.

I'm using Mingw-builds 4.8.1 on Windows 7 (OS is 64-bit, Mingw is 32-bit) and the Rust 0.11.0 release.

[vadimcn]

We used to let gcc add the appropriate version of libgcc to linker command line, so the -static-libgcc switch would work. Later, however, an explicit #[link("gcc_s")] was added in libunwind.rs, which takes precedence.
I suppose this could be reverted, however, if we aspire to invoke 'ld' directly, this wouldn't be a long-term solution. To do this properly, we may have to add some support for conditional linking, similar to --cfg.