Fix non-arm GCC targets

[rsaxvc]

Part of #32, this fixes compiling CMSIS-DSP on non-ARM and old ARM(like ARM11) GCC .

[JonatanAntoni]

Not sure if I get this. Why would one include CMSIS at all if not compiling for a (supported) Arm target? Instead of conditionally exclude content from the compiler header file probably we need to include something totally different.

cc: @christophe0606

[rsaxvc]

For ArduPilot's simulator I'd like to be able to run the same Fourier transform on logged sensor data and get the same result. Doesn't need to run fast.

On Raspberry Pi, The least common instruction set is ARM1176JZFS, where the heavy lifting can be done with much the same DSP instructions as Cortex-M4.

For context: ARM-software/CMSIS_5#490

[JonatanAntoni]

Hi @rsaxvc,

That's fair request. I still don't get the point why you'd want to include the CMSIS GCC Compiler header which provides Arm-specific defines on a non-arm target? Wouldn't it be cleaner approach to not include this header at all?

[rsaxvc]

Sorry for the confusion, for the PC simulator, it is enough to build CMSIS-DSP in host mode and skip CMSIS6 entirely.

[JonatanAntoni]

No worries. Is this PR still relevant for older ARM targets such as ARM11? Basically, CMSIS 6 doesn't claim support for anything older than Armv6-M (Cortex-M0). Still, if with simple changes we can allow your use case that'd be perfectly fine.

[rsaxvc]

No longer relevant since I can build CMSIS-DSP in host mode - I'll close it