`--linker lld|mold` not reliably applied at link time

[GaspardKirira]

Description

When using --linker lld or --linker mold, the selected linker was not always
used during the final link step. Linker flags were incorrectly propagated via
CMAKE_C_FLAGS / CMAKE_CXX_FLAGS, leading to unreliable behavior and CMake
warnings about unused variables.

Impact

• Fast linkers (lld, mold) sometimes ignored
• Inconsistent build results
• Noisy CMake warnings

Resolution

• Fast linker flags are now applied via:
  • CMAKE_EXE_LINKER_FLAGS
  • CMAKE_SHARED_LINKER_FLAGS
  • CMAKE_MODULE_LINKER_FLAGS
• Removed incorrect propagation through compile flags

Result

Linker selection is now deterministic, correct, and warning-free.