Accept unnamed array in gdb.ada/limited-length.exp

Some compiler changes I'm working on cause a regression in
gdb.ada/limited-length.exp -- with the changes, the array type is
nameless and so is not mentioned in the max-value-size error message.

Because the array type is nameless in the source code, this seems like
an improvement to me, and so this patch changes the test to accept
either form.