[gdb/testsuite] Require istarget x86* in gdb.reverse/step-indirect-call-thunk.exp

[gdb/testsuite] Require istarget x86* in gdb.reverse/step-indirect-call-thunk.exp

On aarch64-linux, I run into:
...
Running gdb.reverse/step-indirect-call-thunk.exp ...
gdb compile failed, gcc: error: unrecognized command line option \
  '-mindirect-branch=thunk'; did you mean '-findirect-inlining'?
gcc: error: unrecognized command line option '-mfunction-return=thunk'; \
  did you mean '-Wfunction-elimination'?
UNTESTED: gdb.reverse/step-indirect-call-thunk.exp: failed to prepare
...

Fix this by requiring istarget "x86*", similar to what was added in
gdb.base/step-indirect-call-thunk.exp by commit 43127ae5714 ("Fix
gdb.base/step-indirect-call-thunk.exp"), such that we have instead:
...
UNSUPPORTED: gdb.reverse/step-indirect-call-thunk.exp: require failed: \
  istarget "x86*
...

Tested on x86_64-linux and aarch64-linux.