package/llvm: remove $ORIGIN/../lib from RPATH

package/llvm: remove $ORIGIN/../lib from RPATH

AddLLVM.cmake adds $ORIGIN/../lib to the RPATH of llvm binaries.  This
causes a problem when llvm-config built for the host, installed in
STAGING_DIR, is executed under the following conditions:

* Target architecture same as host architecture (normally x86_64)
* Target's libc different from host's libc (normally glibc)

llvm-config will try to link with the target's libc, resulting in:

./llvm-config: error while loading shared libraries: libc.so.0: cannot open shared object file: No such file or directory.

To avoid this, we simply add pass
-DCMAKE_INSTALL_RPATH="$(HOST_DIR)/lib" in HOST_LLVM_CONF_OPTS.

Link to discussion:

  http://lists.busybox.net/pipermail/buildroot/2018-April/218627.html

Fixes:

  http://autobuild.buildroot.net/results/b81c12d529c66a028e2297ea5ce1d6930324fa69/

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>