corefx does not build on Ubuntu 17.04

[wfurt]

This is because clang-3.5 is no longer available and system comes with 3.8 by default.
The src/Native/ build script has option to specify different version but that is not exposed to top level scripts (build.sh and build-native.sh)

This is somewhat unpleasant as for example we require minimum 3.9 for arm so available 3.8 should work just fine.

I think there are two possible stages for fix:
dotnet/corefx#1 expose clang option to root build script and make sure it works.
dotnet/corefx#2 detect available clang and use closest desired match similar to way how coreclr does.