Remove C++ dependencies

[vadimcn]

This PR removes Rust's dependency on C++ for exception handling. Instead, it will use the unwind library API directly.

closes #10469

[alexcrichton]

The only thing blocking me from r+'ing this PR is the forceful addition of -shared-libgcc to the command line. See my comments on the code for why.

Other than that though, this is again truly amazing! Fantastic job with explaining everything in the comments, you are a hero!

[vadimcn]

@acrichton: PR updated. BTW, that -shared-libgcc switch may be overridden by --link-args "-static-libgcc"

[vadimcn]

thanks!

[alexcrichton]

I think this just needs to compile the .ll file with -relocation-model=pic

[vadimcn]

Ok, done.

[alexcrichton]

OSX has libgcc_s apparently instead of libgcc.

[vadimcn]

Should I get rid of #[link(name="gcc")] then?

[brson]

I would like to better understand our GPL obligations after this patch. In what scenario will we be statically linking libstcxx? More than just for snapshots now? I suspect that we need to leave in the previous language about the GPL re libstcxx in the snapshots and furthermore may have new obligations depending on where else were doing static linking.

[vadimcn]

@brson: I've removed static linking from this PR on Alex's request. But GPL with exception does allow static linking without any obligations (their FAQ)

[alexcrichton]

I believe that this will need something along the lines of:

// explain that this is what provides the gcc_personality function and libunwind interface in this comment.
#[cfg(target_os = "macos")]
#[link(name = "gcc_s")]
extern {}
#[cfg(not(target_os = "macos"))]
#[link(name = "gcc")]
extern {}

Apparently both ubuntu and freebsd have both libgcc and libgcc_s, unsure of what the difference is.

[brson]

@vadimcn I do see that the GPL runtime exception allows distribution via static linking, but I'm assuming that if we distribute the statically linked binary then we have to include the license. That makes me think we need to leave the language in the "third-party" readme about the library.

[vadimcn]

hope this does it. r?

[alexcrichton]

Can you squash these commits into one?

[vadimcn]

Done.