[gdb/testsuite] Fix gdb.arch/amd64-stap-special-operands.exp for remote host

With test-case gdb.arch/amd64-stap-special-operands.exp and host board
local-remote-host-notty and target board native-gdbserver I run into:
...
(gdb) break -pstap three_arg^M
No probe matching objfile=`<any>', provider=`<any>', name=`three_arg'^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) FAIL: gdb.arch/amd64-stap-special-operands.exp: probe: three_arg: \
  gdb_breakpoint: set breakpoint at -pstap three_arg
...
due to compiling two executables with the same name, and when uploading the
second one from host to build, we run into:
...
Upload from 127.0.0.1 failed, \
  $outputs/gdb.arch/amd64-stap-special-operands/amd64-stap-special-operands: \
  Text file busy.
...

Fix this by making the executable names unique.

Tested on x86_64-linux.