Since commit c26c6ac488 (Link Step: compute effective linker used by the
compiler, 2023-05-27) we used `UNKNOWN` if the linker id is not known.
This convention is not used elsewhere, and logic using the linker id
is easier when `if(CMAKE_C_COMPILER_LINKER_ID)` is false on unknown id.
`cmake_determine_linker_id` is only used for macOS because Apple device
platforms set `_CMAKE_FEATURE_DETECTION_TARGET_TYPE` to `STATIC_LIBRARY`,
but we might as well prepare for them anyway.
Extract the effective linker during the computation of implicit artifacts
delivered by the compiler to the linker.
Define various variables describing the linker:
* CMAKE_<LANG>_COMPILER_LINKER
* CMAKE_<LANG>_COMPILER_LINKER_VERSION
* CMAKE_<LANG>_COMPILER_LINKER_ID
* CMAKE_<LANG>_COMPILER_LINKER_FRONTEND_VARIANT
This is complementary to feature introduced by commit 96a953b1ed
(Add options to specify linker tool, 2023-09-27).
Fixes: #17596, #18209, #25344
Brad King