tcg: Handle dh_typecode_i128 with TCG_CALL_{RET,ARG}_NORMAL

Many hosts pass and return 128-bit quantities like sequential
64-bit quantities.  Treat this just like we currently break
down 64-bit quantities for a 32-bit host.

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