Insert placeholder for FE DE FF E7 opcode

[DTeachs]

This is for issue #3723, since this appears to be a debugger ptrace breakpoint.

[DTeachs]

@ryanmkurtz This is a breakpoint on Nintendo Switch applications on Nvidia Tegra processors.

[GhidorahRex]

@GameCubeGBA I'll raise this as a discussion. At this point the issue is that this instruction is "not documented" because it's the aarch32 udf 0xfdef instruction being used in aarch64 to generate an undefined instruction exception to act as a breakpoint, rather than use the aarch64 udf instruction.

There's still the possibility that something else is going on. aarch64 can support switching to 32-bit mode through changing exception levels. So the processor can be running a 32-bit application, take an exception to a higher exception level and run aarch64 (but the reverse is not true - there's no way to go from base aarch64 to aarch32 by raising an exception, only by returning from an exception).

If you have a reference to this being used somewhere, I'd love to read up on it.