instructions it not clear about sorting stuff need more clarify explanation and some additional example

Description will lead you into O(nlog(n)) solution at most.
It would be helpful to have in the description info that there is a neat O(n) math solution to the problem.

CoffeeScript translation

good kata, but instruction should be clearer regarding sorting

Sample tests for C and NASM have a minor bug, if you get the answer wrong for the array of length 0, you crash with a SIGSEGV. (At first I thought I may have messed up seriously somewhere lol).
This seems to be because you slip into an unwanted loop here:

while (--n) // RIP when n == 0
    pos += sprintf(s+pos, "%s%i", p, *a++);

RISCV Translation

No random tests (not enough tests) and for example your solution doesn't work for groupDuplicatesAndSort([12, 12, 2, 2, 1, 1]) ;-)