Pull syscall code out of libc and create new libj6. This should eventually become a vDSO, but for now it can still be a static lib. Also renames all the _syscall_* symbol names to j6_*
Userspace can now allocte via malloc. This is slightly janky because it relies on a single static handle in the library code.
Moving libc from its separate repo into this one, minor resulting build fixes, and a hacky way to add -I for libc headers in builds.
