Handle cross compilation in clang

[mattgodbolt]

Can specify --target but it doesn't like --gcc-toolchain. https://godbolt.org/g/emWpGe for example.

Would be great to have libstdc+ too so ARM etc could use it (doesn't want to use gcc)

[partouf]

Considering we've had separately configured clang compilers for other architectures, I suggest we let this one rest unless we discover more need for this.