PCH: Avoid Apple-specific architecture flags on other platforms

Since commit f593b354da (PCH: Add support for multi architecture iOS
projects, 2020-04-02, v3.18.0-rc1~414^2) the `OSX_ARCHITECTURES` target
property (and corresponding `CMAKE_OSX_ARCHITECTURES` variable) affects
flags on non-Apple platforms by accident.  Add a missing condition to
avoid this.

Fixes: #21072
1 file changed