[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/main.cpp

@@ -18,8 +18,8 @@ extern "C" {
     void _get_init(size_t *initc, struct j6_init_value **initv);
 }
 
-extern "C" int
-driver_main(int argc, const char **argv, const char **env, const j6_init_args *init)
+int
+driver_main(unsigned argc, const char **argv, const char **env, const j6_init_args *init)
 {
     j6_log("fb driver starting");
 
@@ -130,5 +130,3 @@ driver_main(int argc, const char **argv, const char **env, const j6_init_args *i
     j6_log("fb driver done, exiting");
     return 0;
 }
-
-int main () __attribute__ ((weak, alias ("driver_main")));

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"]

src/user/testapp/main.cpp

@@ -11,10 +11,6 @@ char inbuf[1024];
 extern j6_handle_t __handle_sys;
 j6_handle_t endp = j6_handle_invalid;
 
-extern "C" {
-    int main(int, const char **);
-}
-
 extern j6_handle_t __handle_self;
 
 constexpr uintptr_t stack_top = 0xf80000000;