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.
Linus and many other people hate C++ for having strange compiler rules and a kitchen sink approach. Ada, for a long time, was linked to a proprietary government supplied compiler, and also did not have many volunteer developers available. Cyclone was garbage collected, and Linus prefers not to deal with garbage collecting. I don't think he's ever commented on D, it's an obscure language.
(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.