package/llvm: add llvm RTTI option

package/llvm: add llvm RTTI option

Allow to build LLVM with run-time type information as this feature is
needed (for example) by mesa3d's Gallium Nouveau driver or the Clover
OpenCL state tracker when llvm support is enabled in mesa3d.

While we only care about RTTI support in the target, we also need to
enable it in the host LLVM, so that llvm-config gives the proper
result.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>