Change __HIP_ENABLE_PCH to be an option

Currently all the logic checks if __HIP_ENABLE_PCH is set, so setting it
to OFF will still enable PCH.

Switching __HIP_ENABLE_PCH to be a CMake option will enable correct
build behaviour.

Change-Id: I77a663589ffb5f2595c6ad3d144eb9466adcf250

rocclr/CMakeLists.txt

@@ -156,7 +156,7 @@ endif()
 
 # Short-Term solution for pre-compiled headers for online compilation
 # Enable pre compiled header
-if(${__HIP_ENABLE_PCH})
+if(__HIP_ENABLE_PCH)
     execute_process(COMMAND sh -c "${CMAKE_CURRENT_SOURCE_DIR}/../bin/hip_embed_pch.sh ${LLVM_DIR}")
     add_definitions(-D__HIP_ENABLE_PCH)
 endif()
@@ -223,7 +223,7 @@ add_library(device INTERFACE)
 target_link_libraries(device INTERFACE host)
 
 # Short-Term solution for pre-compiled headers for online compilation
-if(${__HIP_ENABLE_PCH})
+if(__HIP_ENABLE_PCH)
   target_link_libraries(amdhip64 PRIVATE ${CMAKE_BINARY_DIR}/hip_pch.o)
 endif()