[gdb/testsuite] Fix label reference in implptr-64bit.exp

[gdb/testsuite] Fix label reference in implptr-64bit.exp

When running gdb.dwarf2/implptr-64bit.exp with board cc-with-dwz-m, we run into:
...
dwz: dwz.c:2363: checksum_die: \
  Assertion `\
    ((!op_multifile && !rd_multifile && !fi_multifile) || cu != die_cu (ref)) \
    && (!op_multifile || cu->cu_chunk == die_cu (ref)->cu_chunk)' failed.
cc-with-tweaks.sh: line 218: 13030 Aborted  \
  $DWZ -m ${output_file}.dwz "$output_file" ${output_file}.alt > /dev/null
...
In other words, PR dwz/24170.

The trigger for the dwz PR is when intra-CU references are encoded using
section-relative encoding DW_FORM_ref_addr, but could have been encoded using
CU-relative encoding DW_FORM_ref4.

Fix the intra-CU '%' label reference in implptr-64bit.exp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-06-24  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/implptr-64bit.exp: Fix intra-CU '%' label reference.