[libc] Update install directory for offloading versions of libraries (llvm#83592)

Summary:
These currently go to `lib/`, but if the user has `lib/<triple>` they
should go there because it's always searched first.

Author: jhuber6

libc/CMakeLists.txt

@@ -232,7 +232,7 @@ else()
     set(LIBC_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
   endif()
   if(LIBC_TARGET_OS_IS_GPU)
-    set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
+    set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LIBC_TARGET_TRIPLE})
   else()
     set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR})
   endif()

libc/lib/CMakeLists.txt

@@ -77,9 +77,13 @@ install(
 )
 
 if(LIBC_TARGET_OS_IS_GPU)
+  set(gpu_install_dir lib${LLVM_LIBDIR_SUFFIX})
+  if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
+    set(gpu_install_dir lib${LLVM_LIBDIR_SUFFIX}/${LLVM_HOST_TRIPLE})
+  endif()
   install(
     TARGETS ${added_gpu_archive_targets}
-    ARCHIVE DESTINATION lib${LLVM_LIBDIR_SUFFIX}
+    ARCHIVE DESTINATION ${gpu_install_dir}
     COMPONENT libc
   )
   foreach(file ${added_gpu_bitcode_targets})