Switching to using our own flatcc compiler library. (#7468)

We were already doing our own runtime library slicing, and we have to do our own compiler build for bazel, so this is consistent and lets us avoid including their cmake file (and the issues that creates).
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a2b44d9..2d937d1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -434,17 +434,17 @@
   add_subdirectory(third_party/cpuinfo EXCLUDE_FROM_ALL)
 endif()
 
-iree_set_flatcc_cmake_options()
 add_subdirectory(build_tools/third_party/flatcc EXCLUDE_FROM_ALL)
-add_subdirectory(third_party/flatcc EXCLUDE_FROM_ALL)
 
 add_subdirectory(third_party/vulkan_headers EXCLUDE_FROM_ALL)
 
 # TODO(scotttodd): Iterate some more and find a better place for this.
 if (NOT CMAKE_CROSSCOMPILING)
-  install(TARGETS iree-flatcc-cli
-          COMPONENT iree-flatcc-cli
-          RUNTIME DESTINATION bin)
+  install(
+    TARGETS iree-flatcc-cli
+    COMPONENT iree-flatcc-cli
+    RUNTIME DESTINATION bin
+  )
 endif()
 
 if(IREE_BUILD_COMPILER)