[libc++] Use the __strtoNUM functions from __locale instead of the old API (#118029)

The commit where I switched from using the old locale base API to the
new functions defined inside the __locale namespace forgot to update
references to the strtoNUM functions. This patch fixes that.

GitOrigin-RevId: 81c88135e402d3dfee7387acef86b173132cfb64
1 file changed