Remove the C version of HermitCore and let the Rust version stand on its own feet.
* Stub out all official syscalls in syscall.rs. * Linking against LwIP and libpthread currently needs more exports of the C version, which have been moved to dummies.rs. They can hopefully be replaced by official syscalls in the future. * Supporting Go user programs needs the exported variadic function makecontext(). This cannot be implemented in Rust yet. However, the implementation of the HermitCore C version has been moved into an independent library libgosupport. * libkern is required for LwIP, so build it again.