[libj6] Allow driver_main instead of main for accepting extra arguments

Clang will complain if main() is not declared with 0, 2, or 3 arguments.
In order to allow an extra 4th parameter, a new weak main() symbol which
just jumps to driver_main is defined, and _start passes the extra init
pointer to main.

Additionally, libc's crt0.s _start is made weak, and a matching
_libc_crt0_start symbol is defined for implementations that wish to
override _start but still call libc's _start. (Will be used by init.)

src/libraries/j6/init.s

@@ -0,0 +1,6 @@
+extern driver_main
+global main:function weak (main.end - main)
+main:
+    jmp driver_main
+main.end:
+

src/libraries/j6/j6.module

@@ -5,6 +5,7 @@ j6 = module("j6",
     deps = [ "util" ],
     sources = [
         "init.cpp",
+        "init.s",
         "protocol_ids.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",

src/libraries/j6/j6/init.h

@@ -16,6 +16,9 @@ struct j6_init_args
 /// Find the first handle of the given type held by this process
 j6_handle_t j6_find_first_handle(j6_object_type obj_type);
 
+/// Drivers may use driver_main instead of main
+int driver_main(unsigned, const char **, const char **, const j6_init_args *);
+
 #ifdef __cplusplus
 } // extern "C"
 #endif