Fix dynarec crashes on 3DS

Fix dynarec crashes on 3DS

After the dynarec writes new instructions, it has to flush the
instruction and data caches. Some of these flush operations are
privileged on the 3DS, so the clear cache functions have to run
through svcBackdoor. The Nintendo implementation (and CFW
reimplementation) of svcBackdoor has a problem where interrupts and
context switches will cause crashes.

Even though we can disable interrupts in the flush function, there's
still a window of time between svcBackdoor being called and the
function being run where an interrupt will corrupt the stack.

Luma3DS implemements a svcCustomBackdoor call we can use that also
runs a function in supervisor mode, but uses an implementation that
avoids this problem.