Crate compiler_builtins

Our own compiler_builtins.

Rust provides compiler_builtins as a port of LLVM’s compiler-rt. Since we do not need the vast majority of them, we avoid the dependency by providing this file.

At the moment, some builtins are required that should not be. For instance, core has 128-bit integers functionality which we should not be compiling in. We will work with upstream core to provide feature flags to disable the parts we do not need. For the moment, we define them to panic! at runtime for simplicity to catch mistakes, instead of performing surgery on core.o.

In any case, all these symbols are weakened to ensure we do not override those that may be provided by the rest of the kernel.