TIC6X/GAS: Work around array bounds compilation error

Work around a compilation error:

.../gas/config/tc-tic6x.c: In function 'tic6x_start_unwind_section':
.../gas/config/tc-tic6x.c:4632:17: error: offset '16' outside bounds of constant string [-Werror=array-bounds]
 4632 |       text_name += strlen (".gnu.linkonce.t.");
      |       ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

observed with GCC 10.