[OpenMP] Make libomptarget link against libomp

In `libomptarget` we use a couple of functions from `libomp`, but we didn't link
`libomptarget` against `libomp`. That will not work on some platforms such
as macOS. A linker error will be encountered because those symbols are not resolved
at link time when building `libomptarget`. This patch simply makes `libomptarget`
link agains `libomp`, makes it a "user" of `libomp`. I think this will not break
the policies between `libomp` and `libomptarget`.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D149617

Author: shiltian

openmp/libomptarget/src/CMakeLists.txt

@@ -31,6 +31,10 @@ add_llvm_library(omptarget
   Support
   Object
 
+  LINK_LIBS
+  PUBLIC
+  omp
+
   NO_INSTALL_RPATH
 )
 target_include_directories(omptarget PRIVATE ${LIBOMPTARGET_INCLUDE_DIR})