Implement yield from C hooks.

Get number of multiple results from C frame.
Add lj_cont_hook: restores multres and dispatch to static ins.
Can use fastcall for lj_dispatch_ins() now.

src/lj_vm.h

@@ -54,6 +54,7 @@ LJ_ASMF void lj_cont_ra(void);  /* Store result in RA from instruction. */
 LJ_ASMF void lj_cont_nop(void);  /* Do nothing, just continue execution. */
 LJ_ASMF void lj_cont_condt(void);  /* Branch if result is true. */
 LJ_ASMF void lj_cont_condf(void);  /* Branch if result is false. */
+LJ_ASMF void lj_cont_hook(void);  /* Continue from hook yield. */
 
 /* Start of the ASM code. */
 LJ_ASMF char lj_vm_asm_begin[];