Move iree/tools/ to tools/. (#9135)

Progress on https://github.com/google/iree/issues/8955.

* Bazel target names: `//iree/tools:iree-opt` -> `//tools:iree-opt`
* CMake target names: `iree::tools::iree-opt` -> `iree-opt`
* Updated docs, scripts, and pipelines to the new source file and build directory paths
diff --git a/build_tools/cmake/build_riscv.sh b/build_tools/cmake/build_riscv.sh
index 26658a7..54c5b61 100755
--- a/build_tools/cmake/build_riscv.sh
+++ b/build_tools/cmake/build_riscv.sh
@@ -78,7 +78,7 @@
   )
 elif [[ "${RISCV_CONFIG?}" == "rv32-baremetal" ]]; then
   args+=(
-    # TODO(#6353): Off until iree/tools are refactored to support threadless config.
+    # TODO(#6353): Off until tools/ are refactored to support threadless config.
     -DIREE_BUILD_TESTS=OFF
     -DRISCV_TOOLCHAIN_ROOT="${RISCV_RV32_NEWLIB_TOOLCHAIN_ROOT?}"
   )