accel/tcg: Merge tcg_exec_init into tcg_init_machine

There is only one caller, and shortly we will need access
to the MachineState, which tcg_init_machine already has.

Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>