Emit correct darwin_x86_64 CPU value if not using Xcode

Previously, if not using the Xcode toolchain, for example because the
locator failed, the standard Unix C++ toolchain was created with a CPU
value of either `darwin_arm64` or `darwin`, depending on the host
architecture. The latter, when appended to
`@local_config_cc//:cc-compiler-` in the toolchain BUILD template,
results in a reference to a non-existent toolchain.

tools/cpp/cc_configure.bzl

@@ -100,7 +100,7 @@ def cc_autoconf_impl(repository_ctx, overriden_tools = dict()):
         ])
         repository_ctx.symlink(paths["@bazel_tools//tools/cpp:empty_cc_toolchain_config.bzl"], "cc_toolchain_config.bzl")
         repository_ctx.template("BUILD", paths["@bazel_tools//tools/cpp:BUILD.empty.tpl"], {
-            "%{cpu}": get_cpu_value(repository_ctx),
+            "%{cpu}": cpu_value,
         })
     elif cpu_value == "freebsd" or cpu_value == "openbsd":
         paths = resolve_labels(repository_ctx, [

tools/cpp/lib_cc_configure.bzl

@@ -183,7 +183,7 @@ def get_cpu_value(repository_ctx):
     arch = repository_ctx.os.arch
     if os_name.startswith("mac os"):
         # Check if we are on x86_64 or arm64 and return the corresponding cpu value.
-        return "darwin" + ("_arm64" if arch == "aarch64" else "")
+        return "darwin" + ("_arm64" if arch == "aarch64" else "_x86_64")
     if os_name.find("freebsd") != -1:
         return "freebsd"
     if os_name.find("openbsd") != -1: