[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:
+