Add a flag to choose whether to enable/disable interrupts around arithmetic mode switches

[MichaelBell]

Sometimes you know that the epiphany can't receive any interrupts while your program is running (e.g. because all interrupts are masked, as is the default).

In these cases I believe it is unnecessary to disable and reenable interrupts around arithmode switches, and doing so sometimes costs noticeable performance in cases where floating point and integer arithmetic is mixed.

Therefore, it would be useful to have a flag to choose whether or not interrupts should be disabled when switching arithmode.