Apple: Merge per-arch sysroot parameters if all are the same

Since commit a9b41195 (Handle multi-arch sysroots on Apple platforms,
2019-07-26, v3.17.0-rc1~287^2), 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

Restore 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.