Revert "Retrieve single arch cpu from cc_toolchain instead of from …

…`apple_fragment` (#694)"

This reverts commit 918a4d6.

swift/internal/xcode_swift_toolchain.bzl

@@ -633,45 +633,14 @@ def _xcode_env(xcode_config, platform):
         apple_common.target_apple_env(xcode_config, platform),
     )
 
-# TODO: Revisit ctx.fragments.apple.single_arch_cpu to see if it returns the
-# correct cpu for the exec platform.
-def _single_arch_cpu(cc_toolchain, platform):
-    """Returns the single effective architecture for the current configuration.
-
-    Args:
-        cc_toolchain: The C++ toolchain from which linking flags and other
-            tools needed by the Swift toolchain (such as `clang`) will be
-            retrieved.
-        platform: The `apple_platform` value describing the target platform
-            being built.
-
-    Returns:
-        A single architecture without the platform name (e.g. `x86_64`).
-    """
-    cpu = cc_toolchain.cpu
-    platform_type = str(platform.platform_type)
-    if platform_type == "ios":
-        if cpu.startswith("ios_sim_"):
-            return cpu[len("ios_sim_"):]
-        else:
-            return cpu[len("ios_"):]
-    if platform_type == "watchos":
-        return cpu[len("watchos_"):]
-    if platform_type == "tvos":
-        return cpu[len("tvos_"):]
-    if platform_type == "macos":
-        return cpu[len("darwin_"):]
-
-    fail("ERROR: Unhandled platform type {}".format(platform_type))
-
 def _xcode_swift_toolchain_impl(ctx):
     apple_fragment = ctx.fragments.apple
     cpp_fragment = ctx.fragments.cpp
     apple_toolchain = apple_common.apple_toolchain()
     cc_toolchain = find_cpp_toolchain(ctx)
 
+    cpu = apple_fragment.single_arch_cpu
     platform = apple_fragment.single_arch_platform
-    cpu = _single_arch_cpu(cc_toolchain, platform)
     xcode_config = ctx.attr._xcode_config[apple_common.XcodeVersionConfig]
 
     target_os_version = xcode_config.minimum_os_for_platform_type(