gen-icount.h: Rename gen_icount_start/end to gen_tb_start/end

gen-icount.h: Rename gen_icount_start/end to gen_tb_start/end

The gen_icount_start/end functions are now somewhat misnamed since they
are useful for generic "start/end of TB" code, used for more than just
icount. Rename them to gen_tb_start/end.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>