[libc] Pull crt0 out into its own module

Sorta. crt0 is a separate module as far as bonnibel is concerned, but it's
still part of the libc module file.

src/libraries/libc/arch/amd64/init.cpp

@@ -0,0 +1,38 @@
+#include <stddef.h>
+
+using cb = void (*)(void);
+extern cb __preinit_array_start;
+extern cb __preinit_array_end;
+extern cb __init_array_start;
+extern cb __init_array_end;
+
+namespace {
+
+void
+run_ctor_list(cb *array, cb *end)
+{
+    if (!array || !end)
+        return;
+
+    size_t i = 0;
+    while (true) {
+        const cb &ctor = array[i++];
+        if (&ctor == end) return;
+        if (ctor) ctor();
+    }
+}
+
+void
+run_global_ctors()
+{
+    run_ctor_list(&__preinit_array_start, &__preinit_array_end);
+    run_ctor_list(&__init_array_start, &__init_array_end);
+}
+
+} // namespace
+
+extern "C"
+void __init_libc()
+{
+    run_global_ctors();
+}