Makefile: Fix macOS cross-compilation

[EllaFoxo]

On macOS, the clang compiler is the default for C++ sources. Therefore, we also require the CROSS_PREFIX to be set here as well.

This updates the Makefile, but also fixes the check-gcc script, so that it understands we are cross compiling with the prefixed readelf binary.

[Arksine]

Thanks. I'm ok with the change to the makefile, however I don't believe the CROSS_PREFIXmakefile variable propagates to the environment, so the change to check-gcc.sh won't change readelf binary. Presumably its fine to use the native readelf to perform the check anyway.

[EllaFoxo]

It does propagate, as I tested the script being called from the Makefile, and it is able to find the readelf binary correctly

[Arksine]

Interesting. My understanding of make is that it doesn't set environment variables unless you explicit use export. When testing on Ubuntu echoing the cross prefix in check-gcc.sh results in an empty string, which is what I would expect.

[EllaFoxo]

I think Makefiles behave differently, unless there is something we're doing different. I'm running the make commands with CROSS_PREFIX="arm-none-eabi-" make menuconfig etc, which might be why it's picked up. I'll look into it further

[Arksine]

Ah, you are explicitly setting the CROSS_PREFIX in the environment prior to running make. Generally that shouldn't be necessary, the CROSS_PREFIX variable is set by the makefile located platform directory.

I suppose that is fine, however I wouldn't expect users to do this. Does MacOS not have a native readelf binary? Or put differently, does the build fail if you omit the CROSS_PREFIX="arm-none-eabi-" from the command?