Support cross-compilation for use with Nerves

[fhunleth]

This moves the Makefile code that queries the system type and CUDA
availability under a guard for whether things are being cross-compiled.
If cross-compiled, then the settings are hardcoded. This should work in
non-Nerves compilation cases too, but those weren't tested.

[josevalim]

Fine by me, thank you! But @polvalente should merge it :)

[fhunleth]

Thanks!

@polvalente In case you're curious, @lawik has been working with the Whisper model and we've been collaborating on nerves-livebook/nerves_livebook#559. Note that Whisper doesn't work on that PR due to a lack of ffmpeg, but much simpler use seems to be working.

[polvalente]

I believe UNAME_S should be given as an optional env flag. Linux is probably always the target, but I don't think we can assert this for sure. Maybe a different name should be used for the env var as it's now gonna be (optionally) exposed.

I also wonder if we should allow NVCC to be provided via the toolchain for systems like the nvidia jetson.

Suggested change

	else
	# Determine settings for cross-compiled builds like for Nerves
	UNAME_S = Linux
	NVCC := $(CXX)
	NVCCFLAGS = $(CFLAGS)
	endif
	else
	# Determine settings for cross-compiled builds like for Nerves
	UNAME_S ?= Linux
	NVCC := $(CXX)
	NVCCFLAGS = $(CFLAGS)
	endif

[polvalente]

Actually, given that this is only used for checking if it's mac or not, we can merge as is. Crosscompiling for mac is highly unlikely IMO

[fhunleth]

The only cross-compilation I've run into that hasn't been Linux is for RTOSs and the "Linux" options seem to work. It might be easier to address when it comes up.

Thanks for merging!