Swift: print a helpful message on macOS ARM

Also remove the tentative way compilation was expected to be fixed on
macOS ARM without really working.

In the future we will create universal binaries (which requires
compiling our prebuilt package for ARM as well), but until then we must
require the developer to pass `--cpu=darwin_x86_64` to the build
command when building on an ARM macOS platform like the M1. This will be
printed out explicitly now if it's not the case.

swift/rules.bzl

@@ -5,11 +5,7 @@ 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

@@ -1,21 +1,18 @@
 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",
-}
-
+# for the moment, we require --cpu=darwin_x86_64 on an ARM macOS
 [
     alias(
         name = name,
-        actual = select({
-            "@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")
-        }),
+        actual = select(
+            {
+                "@bazel_tools//src/conditions:%s" % arch: "@swift_prebuilt_%s//:%s" % (arch, name)
+                for arch in ("linux", "darwin_x86_64")
+            },
+            no_match_error = "Unsupported platform. Support for the ARM macOS platform is still a todo, " +
+                             "please pass --cpu=darwin_x86_64 for the time being",
+        ),
     )
     for name in ("swift-llvm-support", "swift-test-sdk")
 ]