Build missing intrinsics from compiler-rt (C) source code

[japaric]

• Add compiler-rt source code as a submodule. Use rust-lang/compiler-rt
• Add an opt-in Cargo feature for this. We'll only compile the intrinsics from C code if the feature is enabled.

[japaric]

Alternative: we can use libgcc implementations (instead of compiler-rt's) on the platforms where that library is available.

[japaric]

We'll only compile the intrinsics from C code

This requires conditional compilation logic. Simply copy paste the logic from rust-lang/rust's src/libcompiler_builtins/build.rs