Xcode: Fix XCODE_EMBED_FRAMEWORKS when settings differ across targets

In commit 5651901c54 (Xcode: add support for embedding frameworks,
2020-10-24, v3.20.0-rc1~402^2) we incorrectly reused `PBXBuildFile`
instances when the same framework is embedded in multiple targets,
causing target-specific settings to conflict.

Fixes: #26438
8 files changed