accel/tcg: Make cpu-exec-common.c a target agnostic unit

accel/tcg: Make cpu-exec-common.c a target agnostic unit

cpu_in_serial_context() is not target specific,
move it declaration to "internal-common.h" (which
we include in the 4 source files modified).

Remove the unused "exec/exec-all.h" header from
cpu-exec-common.c.  There is no more target specific
code in this file: make it target agnostic.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20230914185718.76241-12-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>