[Linaro:ARM_CI] Add clang support to aarch64 toolchain
Hi @MichaelHudgins Can you please review this PR ? Thank you! @elfringham let me know when you want to get this merged.  Hi @elfringham Any update on this CL? Please. Thank you! @gbaned The agreement here was to merge after 2.13 release. @MichaelHudgins We would like to have this PR merged now please. It adds the toolchain but does not use it by default. This will allow use and testing by other CI systems such as Linaro and ARM. Sorry on the delay, was OOO last week.  Can you please rebase? @MichaelHudgins I cannot see what the failure on the feedback/copybara check is about. Is it something I need to fix? Its from a broken windows build from the last LLVM integrate, so not on your end.  
Add the ability to use clang instead of gcc in the aarch64 toolchain