Revert "[cmake] Use `CMAKE_INSTALL_LIBDIR` too"

This reverts commit f7a33090a91015836497c75f173775392ab0304d.

Unfortunately this causes a number of failures that didn't show up in my
local build.
diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index 424f6fe..a0bf269 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -129,9 +129,9 @@
 
   set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
   set(CMAKE_LIBRARY_OUTPUT_DIRECTORY
-    ${CMAKE_BINARY_DIR}/lib)
+    ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
   set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY
-    ${CMAKE_BINARY_DIR}/lib)
+    ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
 
   set(BACKEND_PACKAGE_STRING "LLVM ${LLVM_PACKAGE_VERSION}")
   set(LLVM_EXTERNAL_LIT "${LLVM_TOOLS_BINARY_DIR}/llvm-lit" CACHE STRING "Command used to spawn lit")