LinkerId: Use empty string for unknown linker id

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.
1 file changed