I don't know about Ada+Spark, Cyclone, D, but, since it can compile to C, you can write Linux kernel modules in Nim without Linus' permission / specific "help" from the Linux kernel team: https://forum.nim-lang.org/t/10303
(Yes, not stock Nim, but in that Araq expresses an interest in that thread in supporting said in stock Nim.)
This is one advantage to a C/C++ target in addition to trying to optimize the actual emitted assembly from gcc/clang/etc. It also helps when LLVM may not support your CPU, but you have a C compiler, such as in some embedded spaces. There are trade-offs, of course, as with everything.
(Yes, not stock Nim, but in that Araq expresses an interest in that thread in supporting said in stock Nim.)
This is one advantage to a C/C++ target in addition to trying to optimize the actual emitted assembly from gcc/clang/etc. It also helps when LLVM may not support your CPU, but you have a C compiler, such as in some embedded spaces. There are trade-offs, of course, as with everything.