[gdb/testsuite] Fix regression in step-indirect-call-thunk.exp with gcc 7

Since commit 43127ae5 ("Fix gdb.base/step-indirect-call-thunk.exp") I run
into:
...
gdb compile failed, gcc: error: unrecognized command line option \
  '-fcf-protection=none'; did you mean '-flto-partition=none'?
UNTESTED: gdb.base/step-indirect-call-thunk.exp: failed to prepare
...

The problem is that -fcf-protection is supported starting gcc 8, but I'm using
system gcc 7.5.0.

Fix this by only adding -fcf-protection=none for gcc 8 and later.

Tested on x86_64-linux, with gcc 7.5.0, 8.2.1 and 12.1.1.