tcg: Implement gvec support for rotate by vector

tcg: Implement gvec support for rotate by vector

No host backend support yet, but the interfaces for rotlv
and rotrv are in place.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
v3: Drop the generic expansion from rot to shift; we can do better
    for each backend, and then this code becomes unused.