[gdb/testsuite] Fix XPASS with gcc-12 in gdb.base/vla-struct-fields.exp

[gdb/testsuite] Fix XPASS with gcc-12 in gdb.base/vla-struct-fields.exp

With gcc-12, I get for test-case gdb.base/vla-struct-fields.exp:
...
(gdb) print inner_vla_struct_object_size == sizeof(inner_vla_struct_object)^M
$7 = 1^M
(gdb) XPASS: gdb.base/vla-struct-fields.exp: size of inner_vla_struct_object
...

Fix this by limiting the xfailing to gcc-11 and earlier.  Also, limit the
xfailing to the equality test.

Tested on x86_64-linux.