[gdb/testsuite] Fix gdb.guile/scm-type.exp with gcc 4.8

[gdb/testsuite] Fix gdb.guile/scm-type.exp with gcc 4.8

With gcc 7.5.0, I get:
...
(gdb) guile (print (type-range (field-type (type-field (value-type \
  (value-dereference f)) "items"))))^M
= (0 0)^M
(gdb) PASS: gdb.guile/scm-type.exp: lang_cpp: test_range: \
  on flexible array member: $cmd
...
but with gcc 4.8.5, I get instead:
...
(gdb) guile (print (type-range (field-type (type-field (value-type \
  (value-dereference f)) "items"))))^M
= (0 -1)^M
(gdb) FAIL: gdb.guile/scm-type.exp: lang_cpp: test_range: \
  on flexible array member: $cmd
...

There's a difference in debug info.  With gcc 4.8.5, we have:
...
 <2><224>: Abbrev Number: 15 (DW_TAG_member)
    <225>   DW_AT_name        : items
    <22b>   DW_AT_type        : <0x231>
 <1><231>: Abbrev Number: 4 (DW_TAG_array_type)
    <232>   DW_AT_type        : <0x105>
 <2><23a>: Abbrev Number: 16 (DW_TAG_subrange_type)
    <23b>   DW_AT_type        : <0x11a>
    <23f>   DW_AT_upper_bound : 0xffffffffffffffff
...
and with gcc 7.5.0, we have instead:
...
 <2><89f>: Abbrev Number: 12 (DW_TAG_member)
    <8a0>   DW_AT_name        : items
    <8a6>   DW_AT_type        : <0x8ac>
 <1><8ac>: Abbrev Number: 17 (DW_TAG_array_type)
    <8ad>   DW_AT_type        : <0x29d>
 <2><8b5>: Abbrev Number: 41 (DW_TAG_subrange_type)
 <2><8b6>: Abbrev Number: 0
...

As mentioned in commit 858c8f2c1b9 "gdb/testsuite: adjust
gdb.python/flexible-array-member.exp expected pattern":
...
Ideally, GDB would present a consistent and documented value for an
array member declared with size 0, regardless of how the debug info
looks like.
...

As in gdb.python/flexible-array-member.exp, change the test to accept the two
values.

Tested on x86_64-linux.