Initial import from flutter/buildroot

This imports the code as-is from flutter/buildroot. This code has been
unchanged since the initial import in
https://github.com/flutter/buildroot/commit/af5d72a1e41672bc07f6ebb9274f9f5b3f835687.

Eventually this repo should be replaced with
external/github.com/libexpat/libexpat which would mirror the upstream
libexpat repo, and a BUILD.gn should be added under GN's secondary/
directory so that that repo can remain a pristine copy of upstream.

Bug: https://github.com/flutter/flutter/issues/39532
Change-Id: I61622a1c0e1bc2a92359a37722d2ccbb5ec3d93e
Reviewed-on: https://flutter-review.googlesource.com/c/third_party/expat/+/12700
Reviewed-by: Kaushik Iska <kaushikiska@google.com>