tcg: Move vec_gen_* declarations to tcg-internal.h

These are used within tcg-op-vec.c and tcg/host/tcg-target.c.inc.
There are no uses outside tcg/.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20231029210848.78234-7-richard.henderson@linaro.org>