assuming your string is in A1, the following array function does the trick:
{=TEXTJOIN("",TRUE,MID(A1,ROW(INDIRECT("1:"&LEN(A1)))*-1+LEN(A1)+1,1))}
(don't enter the curly brackets, use control-shift-enter instead)
to break it down, assuming "abc" in A1:
INDIRECT("1:"&LEN(A1)) returns 1:3...