Add some FFI testing and more improvements to sysv abi.

Add support for integer return and floating point return variants, as
well as arguments on the stack. Start flushing out struct arguments.

Still needed:
- structs (packed and unpacked)
- complex numbers
- long doubles
- MASM alternative for windows (you can technically use sysv abi on
  windows)
- more calling conventions.

src/core/util.h

@@ -31,6 +31,8 @@
 
 #include <stdio.h>
 #include <errno.h>
+#include <stddef.h>
+#include <alloca.h> /* for ffi */
 
 #if !defined(JANET_REDUCED_OS) || !defined(JANET_SINGLE_THREADED)
 #include <time.h>