[ffi] Support inline array of variable length

[dcharkes]

Right now we're disallowing inline arrays of length 0.

I believe when I added inline arrays, I read that length 0 is undefined behavior, so I disallowed length 0.

Declaring zero-length arrays is allowed in GNU C as an extension. A zero-length array can be useful as the last element of a structure that is really a header for a variable-length object

https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

_Originally posted by @jpnurmi in dart-lang/native#450

We should double check.

[dcharkes]

And on Windows apparently variable length can be size 1 (instead of 0).

• [DartError] RangeError: Invalid value: Only valid value is 0: 1; #0 Array._checkIndex (dart:ffi-patch/ffi_patch.dart:349:7)#1 main (package:swiftsoft/main.dart)<asynchronous suspension> native#1200 (comment)