[gdb/testsuite] Fix gdb.reverse/step-indirect-call-thunk.exp with -fcf-protection

On Ubuntu 22.04.1 x86_64, I run into:
...
gdb.reverse/step-indirect-call-thunk.c: In function 'inc':^M
gdb.reverse/step-indirect-call-thunk.c:22:1: error: '-mindirect-branch' and \
  '-fcf-protection' are not compatible^M
   22 | {                /* inc.1 */^M
      | ^^M
...

Fix this by forcing -fcf-protection=none, if supported.

Tested on x86_64-linux.