gvisor: Explicitly register C++ toolchains for cross-compilation.

Bazel >= 5 seems to no longer implicitly recognize coral_crosstool's
C++ toolchains when looking through the possible set of toolchains to use for
compiling C++ (`@bazel_tools//tools/cpp:toolchain_type`).

This change explicitly registers these toolchains within `WORKSPACE`.

PiperOrigin-RevId: 492011138

BUILD

@@ -145,6 +145,28 @@ go_path(
     ],
 )
 
+# CC toolchain targets for cross-compilation.
+# Required to be explicitly specified in bazel >= 5.
+toolchain(
+    name = "cc_toolchain_k8",
+    target_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    toolchain = "@crosstool//:cc-compiler-k8",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
+)
+
+toolchain(
+    name = "cc_toolchain_aarch64",
+    target_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:aarch64",
+    ],
+    toolchain = "@crosstool//:cc-compiler-aarch64",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
+)
+
 # gazelle is a set of build tools.
 #
 # To update the WORKSPACE from go.mod, use:

WORKSPACE

@@ -137,6 +137,7 @@ http_archive(
 load("@coral_crosstool//:configure.bzl", "cc_crosstool")
 
 cc_crosstool(name = "crosstool")
+register_toolchains("//:cc_toolchain_k8", "//:cc_toolchain_aarch64")
 
 # Load protobuf dependencies.
 http_archive(