Added compiler-rt submodule.
Build compiler-rt and link it to all crates, similarly to morestack.
auto merge of #12027 : vadimcn/rust/compiler-rt, r=alexcrichton

This is an attempt to remove some more of Rust's dependencies on libgcc and replace it with LLVM's compiler-rt lib.  I've added compiler-rt as a submodule and changed libstd to link with it.  
As far as I could verify, after this change, the only symbols still imported by std from libgcc are the stack unwinding functions.   Other crates, however, still picked up symbols from libgcc, not from libstd, as I had hoped.  So linking definitely requires some work. 

I've only tested this on windows, 32-bit linux and android and fully expect it to fail on other platforms. Patches are welcome.