Stop asserting that threads are not created during deferred ctors

It's valid for constructors to create threads, so we should allow it.
A potentially problematic case (already handled by locking the deferred
constructor list) is where the constructor calls dlopen and triggers
further constructor calls, so add test coverage for it, The new test is
just a copy of filc/tests/concurrentloadstress with the body of main()
moved to a constructor.

Author: pcc

filc/tests/concurrentloadstress3/gctor.c

@@ -0,0 +1,17 @@
+#include <stdfil.h>
+#include <unistd.h>
+#include <sched.h>
+#include <stdbool.h>
+
+static void foo(void) __attribute__((constructor));
+
+static void foo(void)
+{
+    zprintf("gctor: gonna sleep a bit.\n");
+    sched_yield();
+    zprintf("gctor: done.\n");
+}
+
+void bar(void)
+{
+}

filc/tests/concurrentloadstress3/generate-manifest.rb

@@ -0,0 +1,9 @@
+puts "libraries:"
+1000.times {
+    | index |
+    puts "  gctor#{index}.so:"
+    puts "    isBundle: true"
+    puts "    files:"
+    puts "      - gctor.c"
+}
+

filc/tests/concurrentloadstress3/loadgctor.c

@@ -0,0 +1,53 @@
+#include <stdfil.h>
+#include <dlfcn.h>
+#include <pthread.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+#define N 1000
+
+static void do_stuff(const char* name, const char* so, const char* sym)
+{
+    zprintf("%s: loading %s\n", name, so);
+    void* lib = dlopen(so, RTLD_LAZY | RTLD_LOCAL);
+    zprintf("%s: lib = %p\n", name, lib);
+    if (!lib)
+        zprintf("%s: error: %s\n", name, dlerror());
+    zprintf("%s: dlsymming.\n", name);
+    void (*bar)(void) = dlsym(lib, sym);
+    zprintf("%s: %s = %p\n", name, sym, bar);
+    if (!bar)
+        zprintf("%s: error: %s\n", name, dlerror());
+    zprintf("%s: calling %s.\n", name, sym);
+    bar();
+}
+
+static void* thread_main(void* arg)
+{
+    unsigned i;
+    for (i = N / 2; i--;) {
+        do_stuff(
+            "thread",
+            zasprintf("filc/test-output/concurrentloadstress3/gctor%u.so", i + N / 2),
+            "bar");
+    }
+    return NULL;
+}
+
+int main()
+{
+    return 0;
+}
+
+__attribute__((constructor)) static void foo(void)
+{
+    pthread_t t;
+    pthread_create(&t, NULL, thread_main, NULL);
+
+    unsigned i;
+    for (i = N / 2; i--;)
+        do_stuff("main", zasprintf("filc/test-output/concurrentloadstress3/gctor%u.so", i), "bar");
+
+    pthread_join(t, NULL);
+}
+