[LLVM] Fix incorrect GPU triple detection for runtimes builds

Summary:
This block of code is used to prevent a GPU-based cross compiling build
from taking incompatible arguments. However this incorrectly used the
LLVM default triple instead of the runtimes target. Fix that so the bots
can continue to default the triple to NVPTX.

Author: jhuber6

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -120,10 +120,10 @@ if( LLVM_ENABLE_ASSERTIONS )
   endif()
 endif()
 
-# If we are targeting a GPU architecture we want to ignore all the standard
-# flag handling.
-if("${LLVM_DEFAULT_TARGET_TRIPLE}" MATCHES "^amdgcn" OR
-   "${LLVM_DEFAULT_TARGET_TRIPLE}" MATCHES "^nvptx64")
+# If we are targeting a GPU architecture in a runtimes build we want to ignore
+# all the standard flag handling.
+if("${LLVM_RUNTIMES_TARGET}" MATCHES "^amdgcn" OR
+   "${LLVM_RUNTIMES_TARGET}" MATCHES "^nvptx64")
   return()
 endif()