tcg: Implement gvec support for rotate by immediate

tcg: Implement gvec support for rotate by immediate

No host backend support yet, but the interfaces for rotli
are in place.  Canonicalize immediate rotate to the left,
based on a survey of architectures, but provide both left
and right shift interfaces to the translators.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>