[LLVM][NFC] Ignore including the GNUInstallDirs on the GPU (#83910)

Summary:
I've begun treating GPU runtimes builds as cross-compiling with the LLVM
infrastructure. However, we include a lot of random stuff that the GPU
build isn't prepared to handle. This currently emits a warning, and
while it's not striclty necessary, is annoying. This patch suppresses it
by not including the standard GNU install directory resources when used
from the GPU.

runtimes/CMakeLists.txt

@@ -151,6 +151,12 @@ endif()
 # Avoid checking whether the compiler is working.
 set(LLVM_COMPILER_CHECKED ON)
 
+# This can be used to detect whether we're targeting a GPU architecture.
+if("${LLVM_RUNTIMES_TARGET}" MATCHES "^amdgcn" OR
+   "${LLVM_RUNTIMES_TARGET}" MATCHES "^nvptx64")
+  set(LLVM_RUNTIMES_GPU_BUILD ON)
+endif()
+
 # Handle common options used by all runtimes.
 include(AddLLVM)
 include(HandleLLVMOptions)