It seems that dosfstools is supposed to create /usr/sbin/mkdosfs as a symlink to /usr/sbin/mkfs.fat but does not. True for both 4.0-r1 (stable) and 4.1 (~amd64). I have not looked into the code to see why it's missing, but I did not see any errors during the emerge.
Please attach the full build.log of the dosfstools build to this bug and post your "emerge --info".
Apologies - it wasn't clear to me I needed to set the compat USE flag to get the (legacy) symlinks. (not sure if invalid or worksforme would be best resolution)