This fixes Windows build on goma.

Rolls in buildroot change https://github.com/flutter/buildroot/pull/406 https://github.com/flutter/engine/pull/21884 by stuartmorgan which removes the /FC flag in Windows builds. That flag is incompatible with upstream goma changes.

See: https://source.chromium.org/chromium/chromium/src/+/2e6d17c6948b2ca1e4dbd6bf15fcb52d32fa338d

This reverts commit 7959d80c.

- Build OCMock as a dylib for iOS tests
- Set install_name for ios_flutter_test and ocmock dylibs
- Copy and sign dylibs during build process