vcc: Track the built-in subs which static calls originate from

We use another "method" bitmask to record the built-in subs from which
static calls originate.

This allows us to indentify those subs which are called from
housekeeping only, allowing us to selectively disable compiler
optimizations.

VGC diff for the example from
https://github.com/varnishcache/varnish-cache/issues/3545#issue-824634168

```diff
@@ -2242,7 +2278,7 @@

 #define END_ if (*ctx->handling) return

-void v_matchproto_(vcl_func_f)
+void v_dont_optimize v_matchproto_(vcl_func_f)
 VGC_function_a(VRT_CTX, enum vcl_func_call_e call,
     enum vcl_func_fail_e *failp)
 {
```

Fixes #3545

lib/libvcc/vcc_compile.c

@@ -191,7 +191,9 @@ vcc_EmitProc(struct vcc *tl, struct proc *p)
 		maskcmp = "&";
 	}
 
-	if ((mask & VCL_MET_TASK_H) && (mask & ~VCL_MET_TASK_H) == 0)
+	if (dyn == 0 &&
+	    (p->calledfrom &  VCL_MET_TASK_H) != 0 &&
+	    (p->calledfrom & ~VCL_MET_TASK_H) == 0)
 		cc_adv = "v_dont_optimize ";
 	else
 		cc_adv = "";
@@ -210,6 +212,7 @@ vcc_EmitProc(struct vcc *tl, struct proc *p)
 	Fh(tl, 1, "\t.n\t\t= %d,\n", nsub);
 	Fh(tl, 1, "\t.nref\t\t= %d,\n", p->sym->nref);
 	Fh(tl, 1, "\t.called\t\t= %d\n", p->called);
+	Fh(tl, 1, "\t// calledfrom\t  0x%x\n", p->calledfrom);
 	Fh(tl, 1, "}};\n");
 
 	if (dyn) {

lib/libvcc/vcc_compile.h

@@ -213,6 +213,7 @@ struct proc {
 	unsigned		called;
 	unsigned		active;
 	unsigned		okmask;
+	unsigned		calledfrom;
 	struct token		*return_tok[VCL_RET_MAX];
 	struct vsb		*cname;
 	struct vsb		*prologue;

lib/libvcc/vcc_xref.c

@@ -207,6 +207,7 @@ vcc_CheckActionRecurse(struct vcc *tl, struct proc *p, unsigned bitmap)
 			vcc_ErrWhere(tl, pc->t);
 			return (1);
 		}
+		pc->sym->proc->calledfrom |= p->calledfrom;
 		pc->sym->proc->called++;
 		pc->sym->nref += u;
 		if (vcc_CheckActionRecurse(tl, pc->sym->proc, bitmap)) {
@@ -233,10 +234,12 @@ vcc_checkaction(struct vcc *tl, const struct symbol *sym)
 	AN(p);
 	AN(p->name);
 
-	if (p->method == NULL)
+	if (p->method == NULL) {
 		bitmap = ~0U;
-	else
+	} else {
 		bitmap = p->method->ret_bitmap;
+		p->calledfrom = p->method->bitval;
+	}
 
 	if (! vcc_CheckActionRecurse(tl, p, bitmap))
 		return;