clang: globally disable build-time RPATHs for reproducibility

clang: globally disable build-time RPATHs for reproducibility

Various bits of the LLVM project set the build RPATH in a way that means
we have non-deterministic binaries even though we should always be using
relative paths.

This clearly is not working as some of the binaries get rewritten on
install and have large string paddings that correlate with erased build
paths.

So that we don't have to disable RPATHs in every recipe, just do it once
in the common include file and remove the existing recipe-specific
assignments that are now not needed.

Signed-off-by: Ross Burton <ross.burton@arm.com>