[libc] Call global ctors in user code

This change adds a new __init_libc function which calls all the global
ctors in .init_array, and is called from _start.

src/libraries/libc/arch/amd64/crt/crt0.s

@@ -1,12 +1,14 @@
 extern main
 extern exit
 extern __init_libj6
+extern __init_libc
 
 global _start:function (_start.end - _start)
 _start:
 	mov rbp, rsp
 	mov rdi, rsp
 	call __init_libj6
+	call __init_libc
 
 	pop rdi
 	mov rsi, rsp

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

@@ -0,0 +1,27 @@
+#include <stddef.h>
+
+using cb = void (*)(void);
+extern cb __init_array_start[0];
+extern cb __init_array_end;
+
+namespace {
+
+void
+run_global_ctors()
+{
+    size_t i = 0;
+    while (true) {
+        const cb &fp = __init_array_start[i++];
+        if (&fp == &__init_array_end)
+            return;
+        fp();
+    }
+}
+
+} // namespace
+
+extern "C"
+void __init_libc()
+{
+    run_global_ctors();
+}

src/user/srv.init/start.s

@@ -1,6 +1,7 @@
 extern main
 extern exit
 extern __init_libj6
+extern __init_libc
 extern _arg_modules_phys
 
 section .bss
@@ -28,6 +29,7 @@ _start:
  	mov rbp, rsp
 	mov rdi, rsp
 	call __init_libj6
+	call __init_libc
 
 	pop rdi
 	mov rsi, rsp