Add more information about configuring toolchain, especially on Apple systems

[memegauste]

Since gcc and build-binutils has various versions, and also macOS requires custom grub build, it's should be documented on which compiler actual KenOS version is built, and how to build whole crosschain.

[memegauste]

And here we are, grub building on macOS gives this error:

ld: malformed universal file: slice content is not mach-o or a static library file '/usr/local/lib/libosxfuse.dylib'

Not sure how to resolve it because I have 13.4.1 XCode version...