mbrola/us2: initial support for the voice

This adds support for the us2 MBROLA voice with all the us2
phonemes supported. [1]

NOTE: For this to be used ot speak generic English transcriptions,
the en-US accent map must be used.

[1] The voice information file lists /Or/ as a rhotic allophone
    of /O/, but this does not work -- diphones like <Or-t> generate
    missing diphone errors.
mbrola/us2: support /@`/ and /3`/

This completes the support for American English transcriptions
using the same mapping as us1.