atbash-cipher: runner treats $a1 as callee-saved register

Line 52 in 8a1b960

move $s6, $a1 # take copy of output value

Line 57 in 8a1b960

lb $s4, 0($a1) # load one byte of the actual

After calling atbash_cipher, runner assumes $a1 is not changed.
As far as I know, parameter registers ($a0~$a3) are caller-saved register.

rna-transcription has the same issue.