c4x86 draft: JIT "compiler", runs tests/arginc.c

1) the c4 VM is replaced by crude x86 JIT compiler;
2) it is able to run `tests/arginc.c` successfully.

No rellocations, no native calls at this point.
relocation WIP
relocation works; test samples;
JIT.md updated
memcmp, exit: relative addressing
small cleanup
c4.c with JIT -> c4x86.c