Apple: merge per-arch sysroot parameters if all are the same
Since !4125 (merged) CMake supports "fat" builds with different
sysroots. Those are passed to the compiler with the
-Xarch_<xyz> parameter.
Unfortunately this breaks the Compiler Cache (ccache) because it does not support those compiler flags: https://github.com/ccache/ccache/blob/v3.7.9/src/ccache.c#L2700-L2705
This change restores the caching ability for certain "fat" build
configurations (e.g. arm64 and armv7) where the sysroot is the
same for all selected architectures and thus a plain -isysroot
parameter could be used.
Backport: release
Topic-rename: apple-merge-same-sysroot