build: llvm libs may not be in system search path, add rpath

build: llvm libs may not be in system search path, add rpath

On my gentoo system, llvm libs are in /usr/lib64/llvm, and llvm-config
--ldflags does not provide the rpath (it does, of course, provide a -L).
This adds the llvm dir to the rpath. It should be harmless if the path
is a system path, and should make things work when it's not.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Tested-by: Emil Velikov <emil.l.velikov@gmail.com>