Swift: build x86_64 on arm64 macOS

This is a temporary workaround. At a later stage we will add
* a Swift/LLVM prebuilt package for arm64
* universal binary for the extractor

swift/rules.bzl

@@ -5,7 +5,11 @@ def _wrap_cc(rule, kwargs):
     _add_args(kwargs, "copts", [
         # Required by LLVM/Swift
         "-fno-rtti",
-    ])
+    ] + select({
+        # temporary, before we do universal merging and have an arm prebuilt package, we make arm build x86
+        "@platforms//os:macos": ["-arch=x86_64"],
+        "//conditions:default": [],
+    }))
     _add_args(kwargs, "features", [
         # temporary, before we do universal merging
         "-universal_binaries",

swift/tools/prebuilt/BUILD.bazel

@@ -1,11 +1,21 @@
 package(default_visibility = ["//swift:__subpackages__"])
 
+#TODO we will be introducing universal binaries at a later stage, when we have both architectures prebuilt for macOS
+# for the moment, we make arm build an x86_64 binary
+_arch_override = {
+    "darwin_arm64": "darwin_x86_64",
+}
+
 [
     alias(
         name = name,
         actual = select({
-            "@bazel_tools//src/conditions:%s" % arch: "@swift_prebuilt_%s//:%s" % (arch, name)
+            "@bazel_tools//src/conditions:%s" % arch: "@swift_prebuilt_%s//:%s" % (
+                _arch_override.get(arch, arch),
+                name,
+            )
             for arch in ("linux", "darwin_x86_64", "darwin_arm64")
         }),
-    ) for name in ("swift-llvm-support", "swift-test-sdk")
+    )
+    for name in ("swift-llvm-support", "swift-test-sdk")
 ]