Change startup ABI for untrusted code to be C-compatible

src/untrusted/stubs/crt1.x

+/*
+ * This is a dummy linker script used as crt1.o.
+ * The actual startup code is just the _start function defined in a library.
+ * We provide this file for two purposes:
+ * 1. To keep with the traditional linking sequence that puts crt1.o first.
+ * 2. To generate references to the main and exit symbols like the real
+ *    startup code would, so that they will be brought in from libraries
+ *    before -lc is encountered in the link.  Otherwise a main defined in
+ *    a library wouldn't be referenced until after that library had already
+ *    been examined, and _start's call would get an undefined reference.
+ */
+
+EXTERN ( main exit )