add fortified implementations of write/pwrite{,64}

These are just based on the read/pread{,64} implementations with the
function calls and error messages adjusted as appropriate. The only
difference is that the buffer parameters are const.

Change-Id: Ida1597a903807f583f230d74bcedffdb7b24fcf6

libc/libc.map

@@ -1337,6 +1337,9 @@ LIBC_N {
     __fread_chk;
     __fwrite_chk;
     __getcwd_chk;
+    __pwrite_chk;
+    __pwrite64_chk;
+    __write_chk;
     getgrgid_r;
     getgrnam_r;
 } LIBC;