Constify target_pid_to_exec_file

Constify target_pid_to_exec_file

This changes target_pid_to_exec_file and target_ops::pid_to_exec_file
to return a "const char *".  I couldn't build many of these targets,
but did examine the code by hand -- also, as this only affects the
return type, it's normally pretty safe.  This brings gdb and gdbserver
a bit closer, and allows for the removal of a const_cast as well.