GNU: Do not use "fat-lto-objects" flags for IPO on Apple platforms

Fat LTO objects contain both traditional object code and the LTO bitcode
IR, but the GNU compiler does not support them on Apple platforms.
A compile error is raised when `-f[no-]fat-lto-objects` flags are used,
so avoid them.

This also implies that static Fortran libraries cannot be built with IPO.

Fixes: #25931
2 files changed