[gdb/testsuite] Use gdb_gcore_cmd in gdb.threads/gcore-thread.exp

I noticed a plain gcore command in test-case gdb.threads/gcore-thread.exp:
...
    gdb_test "gcore $core0file" "Saved corefile .*" \
      "save a zeroed-threads corefile"
...

Use gdb_gcore_cmd instead.

Tested on x86_64-linux.