This is a follow-up ticket/issue from ICU-20434.
From the previous ticket/issue:
It seems that Cygwin 3.0.0 was released on Saturday, Feb 16, and now the ICU4C AppVeyor CI builds are failing. (Previously, the AppVeyor builds used Cygwin 2.11.2-1.)
From the build output, it looks like pkgdata is failing with the following error:
Assembly type "-a gcc-mingw64" is unknown.