Try to eliminate "new" libstdc++ dependencies in rt and rustllvm

[graydon]

#1345 ran into a number of problems building on RHEL5. The endgame seems to involve ripping out various uses of iostreams from librt and librustllvm (possibly llvm itself). Unclear how much work is involved here.

There may be other "next" problems on RHEL5 after this is fixed. Go step by step, file followup bugs as you find more problems.

[graydon]

non-critical for 0.6, de-milestoning

[bstrie]

Is this obviated by the effort to rewrite the runtime in Rust?

[thestinger]

@bstrie: I don't think so, the problem appears to be LLVM.

I don't think it makes sense to attempt this because these patches are unlikely to ever be accepted upstream, so we would never be able to use the system LLVM.

[bblum]

@graydon, can you elaborate on what changes to llvm itself might be needed here?

tagging rfc, i guess, so we can decide if it's worth attempting.

[thestinger]

LLVM is headed towards switching to C++11, so it would involve a major fork of the project. LLD is already written using C++11 and I think we'll want to be start using it eventually.

http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-January/057787.html

[alexcrichton]

Closing in favor of #10469. I don't think that we'll ever drop the libstdc++ dependency for LLVM, but we should definitely be able to drop it for libstd (tracked in the linked issue).