When building with gcc < 15, CMakeLists.txt unconditionally adds
ime2_kernels.cpp, which fails to compile. FindSMTIME.cmake only defines
RISCV64_SPACEMIT_IME2 when the IME2 instructions are detected, and gcc 14
only has IME1, so ime2_kernels.cpp hits its #error.
This PR fixes it by using IN_LIST to add each kernel source according to
the spec that was actually detected.
