enable_language: Establish target platform identification variables earlier

Brad Kingrequested to merge
brad.king/cmake:target-system-variables into master

Set target platform identification variables like APPLE and LINUX as soon as the target system is identified. This makes them available during toolchain and binutils selection.

Fixes: #23333 (closed)
Fixes: #24975 (closed)
Backport: release

Edited by Brad King

Merge request reports