[gdb/testsuite] Fix xfail in gdb.ada/variant_record_field.exp

On ppc64-linux (debian 14) I run into:
...
(gdb) print p_record^M
$1 = (kind => five, i => <error reading variable: \
  access outside bounds of object>^M
(gdb) gdb_do_cache: get_compiler_info_1 ( c )
get_compiler_info: gcc-15-2-0
gdb_do_cache: get_compiler_info_1 ( c )
FAIL: gdb.ada/variant_record_field.exp: print p_record
...

The test-case contains an xfail for "[gcc_major_version] <= 14", which doesn't
trigger because gcc has version 15.2.0, while gnatmake does have version
14.3.0:
...
$ gcc --version
gcc (Debian 15.2.0-4) 15.2.0
  ...
$ gnatmake --version
GNATMAKE 14.3.0
...

Fix this by using "[gnat_version_compare < 15]" instead.

Tested on ppc64-linux and x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>