[gdb/testsuite] Fix gdb.base/coredump-filter-build-id.exp with older eu-unstrip

[gdb/testsuite] Fix gdb.base/coredump-filter-build-id.exp with older eu-unstrip

On openSUSE Leap 42.3 with eu-unstrip 0.158, we run into:
...
(gdb) PASS: gdb.base/coredump-filter-build-id.exp: save corefile
First line of eu-unstrip: \
  0x400000+0x202000 f4ae8502bd6a14770182382316bc595e9dc6f08b@0x400284 - - [exe]
FAIL: gdb.base/coredump-filter-build-id.exp: gcore dumped mapping with build-id
...

The test expects an actual file name instead of '[exe]', but that only got
introduced with eu-unstrip 0.161.  Before it printed '[exe]' or '[pie]'.

Fix this by updating the regexp.

Tested on x86_64-linux.