tcg: Add generic helpers for saturating arithmetic

No vector ops as yet.  SSE only has direct support for 8- and 16-bit
saturation; handling 32- and 64-bit saturation is much more expensive.

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