[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/user/drv.uefi_fb/uefi_fb.module

@@ -1,6 +1,6 @@
 # vim: ft=python
 
-fb = module("drv.uefi_fb",
+module("drv.uefi_fb",
     targets = [ "user" ],
     deps = [ "libc", "bootproto" ],
     description = "UEFI framebuffer driver",
@@ -11,5 +11,3 @@ fb = module("drv.uefi_fb",
         "screen.cpp",
         "scrollback.cpp",
     ])
-
-fb.variables['asflags'] = ["${asflags}", "-dmain_func=driver_main"]