[OpenMP] Fix multiply installing libomp.so (llvm#93685)

Summary:
The `add_llvm_library` interface handles installing the llvm libraries,
however we want to do our own handling. Otherwise, this will install
into the `./lib` location instead of the `./lib/<target>` one.

openmp/runtime/src/CMakeLists.txt

@@ -177,6 +177,7 @@ else()
   add_llvm_library(omp ${LIBOMP_LIBRARY_KIND} ${LIBOMP_SOURCE_FILES} PARTIAL_SOURCES_INTENDED
     LINK_LIBS ${LIBOMP_CONFIGURED_LIBFLAGS} ${LIBOMP_DL_LIBS}
     LINK_COMPONENTS Support
+    BUILDTREE_ONLY
     )
   # libomp must be a C++ library such that it can link libLLVMSupport
   set(LIBOMP_LINKER_LANGUAGE CXX)