Revert "[OpenMP] Use add_llvm_library to build the target `PluginIn…

…terface` in `plugins-nextgen`"

Breaks cmake regeneration for me:
```
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.cuda.nextgen" which requires target "PluginInterface" that is not in any export set.
CMake Error: install(EXPORT "LLVMExports" ...) includes target "omptarget.rtl.x86_64.nextgen" which requires target "PluginInterface" that is not in any export set.
```

This reverts commit 08c4081.

openmp/libomptarget/plugins-nextgen/common/PluginInterface/CMakeLists.txt

@@ -11,28 +11,24 @@
 ##===----------------------------------------------------------------------===##
 
 # Plugin Interface library.
-add_llvm_library(PluginInterface PluginInterface.cpp GlobalHandler.cpp
-  BUILDTREE_ONLY
+add_library(PluginInterface OBJECT PluginInterface.cpp GlobalHandler.cpp)
 
-  LINK_COMPONENTS
-    Support
+# Define the TARGET_NAME.
+add_definitions("-DTARGET_NAME=PluginInterface")
 
-  LINK_LIBS PRIVATE
-    elf_common
-    MemoryManager
-)
-
-# Define the TARGET_NAME and DEBUG_PREFIX.
-target_compile_definitions(PluginInterface PRIVATE
-  TARGET_NAME="PluginInterface"
-  DEBUG_PREFIX="PluginInterface"
-)
-
-target_include_directories(PluginInterface
-  INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
-  PRIVATE ${LIBOMPTARGET_INCLUDE_DIR}
-)
+# Define the DEBUG_PREFIX.
+add_definitions(-DDEBUG_PREFIX="PluginInterface")
 
 set_target_properties(PluginInterface PROPERTIES
   POSITION_INDEPENDENT_CODE ON
   CXX_VISIBILITY_PRESET protected)
+llvm_update_compile_flags(PluginInterface)
+set(LINK_LLVM_LIBS LLVMSupport)
+if (LLVM_LINK_LLVM_DYLIB)
+  set(LINK_LLVM_LIBS LLVM)
+endif()
+target_link_libraries(PluginInterface INTERFACE ${LINK_LLVM_LIBS} PRIVATE elf_common MemoryManager)
+add_dependencies(PluginInterface ${LINK_LLVM_LIBS})
+
+target_include_directories(PluginInterface INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(PluginInterface PRIVATE ${LIBOMPTARGET_INCLUDE_DIR})