If you want your entire array on a single line, you can write
function string convert2string();
string str1;
str1 = {
$sformatf("m_first :'h%h\n", first),
....
}
$swriteh(str, "%s wdata :%0p\n", str, wdata),
return(str1);
endfunction
Otherwise you will need to use a foreach or for loop to iterate over each element and insert newlines where you want them.