clang: consolidate LLVM_APPEND_VC_REV=OFF

clang: consolidate LLVM_APPEND_VC_REV=OFF

Whilst the change to add TMPDIR to GIT_CEILING_DIRECTORIES should stop
LLVM from embedding git information into the recipes, also disable this
behaviour explicitly.

We do this because it's not just the sha of the source tree but also
the full URL of the repository, which would be an information leak if
an internal git mirror was being used.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>