[LLVM] Only build the GPU loader utility if it has LLVM-libc

Summary:
There were some discussions about this being included by default. I need
to fix this up and codify the use of LLVM libc inside of LLVM. For now,
just turn it off unless the user requested the `libc` GPU stuff. This
matches the old behavior.

Author: jhuber6

llvm/tools/CMakeLists.txt

@@ -9,7 +9,7 @@
 # traversing each directory.
 create_llvm_tool_options()
 
-if(NOT LLVM_COMPILER_IS_GCC_COMPATIBLE)
+if(NOT LLVM_COMPILER_IS_GCC_COMPATIBLE OR NOT LLVM_LIBC_GPU_BUILD)
   set(LLVM_TOOL_LLVM_GPU_LOADER_BUILD OFF)
 endif()