os current_exe using same approach as linux to get always the full absolute path

but in case of failure (e.g. prcfs not mounted) still using
getexecname.

library/std/src/sys/unix/os.rs

@@ -368,6 +368,9 @@ pub fn current_exe() -> io::Result<PathBuf> {
 
 #[cfg(any(target_os = "solaris", target_os = "illumos"))]
 pub fn current_exe() -> io::Result<PathBuf> {
+    if let Ok(path) = crate::fs::read_link("/proc/self/path/a.out") {
+        Ok(path)
+    } else {
         extern "C" {
             fn getexecname() -> *const c_char;
         }
@@ -384,6 +387,7 @@ pub fn current_exe() -> io::Result<PathBuf> {
                 if filename[0] == b'/' { Ok(path) } else { getcwd().map(|cwd| cwd.join(path)) }
             }
         }
+    }
 }
 
 #[cfg(target_os = "haiku")]