add missing implementation of os.getexepath for mac

vlib/os/os.v

@@ -520,8 +520,11 @@ pub fn getexepath() string {
 	}
 
 	$if mac {
-		//panic('getexepath() not impl')
-		return ''
+		mut bufsize := MAX_PATH // if buffer is too small this will be updated with size needed
+		if C._NSGetExecutablePath(result, &bufsize) == -1 {
+			panic('Could not get executable path, buffer too small (need: $bufsize).')
+		}
+		return tos(result, strlen(result))
 	}
 }