vgc: Advise C compiler not optimize housekeeping subs *only*

Before the addition of the okmask, we could only selectively turn off
optimizations for the built-in housekeeping subs (vcl_init {} /
vcl_fini {}), not for any custom subs called from there.

The previous commit changed this and turned off optimizations for any
custom subs called from there.

We pull this tighter again by only turning off optimizations for subs
which can _only_ be called from housekeeping.

Ref f1c47e48
Ref #3230

lib/libvcc/vcc_compile.c

@@ -200,15 +200,8 @@ vcc_EmitProc(struct vcc *tl, struct proc *p)
 	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, "}};\n");
-	/*
-	 * TODO: v_dont_optimize for custom subs called from vcl_init/fini only
-	 *
-	 * Needs infrastructure similar to that in #3163 : custom subs need a
-	 * mask containing the builtin subs calling the custom sub. If that is
-	 * no larger than VCL_MET_TASK_H, we can enable v_dont_optimize
-	 */
 	Fc(tl, 1, "\nvoid %sv_matchproto_(vcl_func_f)\n",
-	   p->method && p->method->bitval & VCL_MET_TASK_H ?
+	   ((mask & VCL_MET_TASK_H) && (mask & ~VCL_MET_TASK_H) == 0) ?
 	   "v_dont_optimize " : "");
 	Fc(tl, 1, "%s(VRT_CTX)\n{\n", VSB_data(p->cname));
 	vsbm = VSB_new_auto();