relibc: add DEBUG marker write before relibc_verify_host

Temporary debugging aid to determine whether relibc_start_v1 is reached
before the init process crashes. Writes a marker to /scheme/debug/no-preserve
at the very start of relibc_start_v1, before any setup or verification.

src/start.rs

@@ -160,6 +160,14 @@ pub unsafe extern "C" fn relibc_start_v1(
         static __init_array_end: extern "C" fn();
     }
 
+    // DEBUG: Earliest possible output before any other init
+    if let Ok(mut f) = unsafe {
+        redox_rt::call::open("/scheme/debug/no-preserve", redox_rt::flag::O_WRONLY, 0)
+    } {
+        let _ = redox_rt::call::write(f.as_raw_fd(), b"START: relibc_start_v1 entered\n");
+        let _ = redox_rt::call::close(f.as_raw_fd());
+    }
+
     // Ensure correct host system before executing more system calls
     unsafe { relibc_verify_host() };