RFC, compiler-rt: Switching x86 linux default compiler-support library from libgcc to compiler-rt

Google has been working to switch our default runtime library for x86-linux to compiler-rt, both to simplify our builds and to possibly enable some interesting new projects.

For situations where builds currently use libgcc.a, our early tests show that libcompiler-rt.builtins.a works reasonably well, with only very minor issues, which we are working to resolve.

Replacing libgcc_s is a little more involved, but we think we will be in a position to do that soon as well. New build rules and such are on their way.

Would there be any objections to switching the default runtime library for linux-x86 to compiler-rt?

Do you mean link LLVM itself against compiler-rt by default, or change
the default Clang chooses when compiling something?

Either way, what would the compatibility story be for distributions
that routinely use GCC? It seems like it would be very easy to break
exception and new/delete ABI compatibility on that route.