[LLVM] Restore LLVM toolchains patch removed by openxla/xla#2013

This patch turns out to be essential for Mac x86->Mac ARM cross compilation to work (jax-ml/jax#15951).

Unfortunately there's no way to test this except manually, because we have no Mac ARM machines in CI at the moment.

PiperOrigin-RevId: 531065713

third_party/llvm/toolchains.patch

@@ -0,0 +1,32 @@
+diff --git a/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel b/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
+index d53167dc618d..de20ef527d05 100644
+--- a/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
++++ b/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
+@@ -19,6 +19,14 @@ licenses(["notice"])
+
+ exports_files(["LICENSE.TXT"])
+
++config_setting(
++    name = "macos_arm64",
++    values = {
++        "apple_platform_type": "macos",
++        "cpu": "darwin_arm64",
++    },
++)
++
+ # It may be tempting to add compiler flags here, but that should be avoided.
+ # The necessary warnings and other compile flags should be provided by the
+ # toolchain or the `.bazelrc` file. This is just a workaround until we have a
+diff --git a/utils/bazel/llvm-project-overlay/llvm/config.bzl b/utils/bazel/llvm-project-overlay/llvm/config.bzl
+index b15ec9e1bb39..0d95a1e4d5c1 100644
+--- a/utils/bazel/llvm-project-overlay/llvm/config.bzl
++++ b/utils/bazel/llvm-project-overlay/llvm/config.bzl
+@@ -89,7 +89,7 @@ os_defines = select({
+ # TODO: We should split out host vs. target here.
+ llvm_config_defines = os_defines + select({
+     "@bazel_tools//src/conditions:windows": native_arch_defines("X86", "x86_64-pc-win32"),
+-    "@bazel_tools//src/conditions:darwin_arm64": native_arch_defines("AArch64", "arm64-apple-darwin"),
++    "//llvm:macos_arm64": native_arch_defines("AArch64", "arm64-apple-darwin"),
+     "@bazel_tools//src/conditions:darwin_x86_64": native_arch_defines("X86", "x86_64-unknown-darwin"),
+     "@bazel_tools//src/conditions:linux_aarch64": native_arch_defines("AArch64", "aarch64-unknown-linux-gnu"),
+     "@bazel_tools//src/conditions:linux_ppc64le": native_arch_defines("PowerPC", "powerpc64le-unknown-linux-gnu"),

third_party/llvm/workspace.bzl

@@ -20,6 +20,7 @@ def repo(name):
             "//third_party/llvm:generated.patch",  # Autogenerated, don't remove.
             "//third_party/llvm:build.patch",
             "//third_party/llvm:mathextras.patch",
+            "//third_party/llvm:toolchains.patch",
             "//third_party/llvm:zstd.patch",
         ],
         link_files = {"//third_party/llvm:run_lit.sh": "mlir/run_lit.sh"},