can run entrypoint on user elf

syscalls via call_abi are still not working

user-apps/calculator/src/main.rs

@@ -1,14 +1,26 @@
 #![no_std]
 #![no_main]
 
+use abi::{Syscall, call_abi};
 use core::panic::PanicInfo;
 
 #[panic_handler]
 fn panic(_info: &PanicInfo) -> ! {
-    loop {} // or call your ABI trap, or `abort()`
+    loop {}
 }
 
 #[unsafe(no_mangle)]
-fn main() {
-    loop {}
+pub extern "C" fn _start() -> ! {
+    loop {
+        for i in 0..300 {
+            for o in 0..300 {
+                let call = Syscall::DrawPixel {
+                    x: i,
+                    y: o,
+                    color: 0,
+                };
+                unsafe { call_abi(&call) };
+            }
+        }
+    }
 }