target-arm/helper.c: tb_flush() on CPU reset

Since target-arm has some CPUState fields for which we take the approach
of baking assumptions about them into translated code and then calling
tb_flush() when the fields change, we must also tb_flush on CPU reset,
because reset is a change of those fields.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>