Add support for crossbuilding to arm-linux-gnueabihf (non-android linux)

[radare]

The current build system permits crosscompilation to linux-arm (but only using the android SDK) and it is not yet possible to specify a generic linux arm target.

[brson]

There is some support for this now but it still needs some llvm patches from #6367

[alexcrichton]

This would be fixed by #7115

[alexcrichton]

Closed in #7115

[alexcrichton]

On second thought, the LLVM patches at in #7115 but this requires more extensive support in the compiler.

[alexcrichton]

On third thought, this appears to be closed by #8220, but feel free to reopen if not!