Hashing a message with a length that isn't a multiple of 8 bits NIST FIPS 180-4. “5.1. Padding the Message.”


  Suppose that the length of the message, $M$, is $l$ bits.


Is there any library that can receive a message of length (in bits) which is not a multiple of $2^3$?
Such libraries most certainly do exist; see here and here for two examples.

A hash implementation that can handle odd-sized bit strings is called "bit-oriented"; most implementations don't bother with it (just because the requirement just doesn't come up that often), however, a few do.