Add nucleotide-count practice exercise

[keiravillekode]

This issue is to add a port to MIPS Assembly of the practice exercise nucleotide-count.

Note that in MIPS, function results are placed in up to 2 registers: $v0 $v1.

Therefore we shouldn't attempt to use 4 registers for the 4 counts.

Using 16 bits for each count, we could pack the 4 counts into 2 registers.

Alternately, we could pass in a pointer to a 4 word destination array.

[github-actions]

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