tcg: Split out tcg init functions to tcg/startup.h

tcg: Split out tcg init functions to tcg/startup.h

The tcg/tcg.h header is a big bucket, containing stuff related to
the translators and the JIT backend.  The places that initialize
tcg or create new threads do not need all of that, so split out
these three functions to a new header.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>