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
Make `CMAKE_LIBRARY_ARCHITECTURE_REGEX` available while determining
compilers before `Platform/{kFreeBSD,GNU}` is loaded.
Follow up commit cc737ae829 (Linux: Provide multiarch library directory
regex earlier, 2023-05-31).