package/clang: new package

package/clang: new package

This patch provides Clang tools and libraries for the host and
libclang for the target.

host-clang is needed to build libclc, which is provided in a follow-up
patch.

We need libclang for the target because it is used by most of OpenCL
implementations.

A later patch in this series will enable Clover, the OpenCL
implementation part of Mesa3D, which requires libclang.

clang-tblgen must be copied to HOST_DIR as it is not installed by
default but is needed for cross-compilation:
http://lists.llvm.org/pipermail/cfe-dev/2015-June/043318.html

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
[Thomas:
 - Add Config.in comment about BR2_TOOLCHAIN_HAS_GCC_BUG_64735
 - Minor reformatting/rewrapping of comments in .mk file]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>