os/mac/hardware: clarify Rosetta 2 situation with a comment.

Expand on #10153 to provide an in-code comment (in case someone goes to
change this in future).

Library/Homebrew/extend/os/mac/hardware.rb

@@ -7,6 +7,13 @@ module Hardware
   def self.oldest_cpu(version = MacOS.version)
     if CPU.arch == :arm64
       :arm_vortex_tempest
+    # TODO: this cannot be re-enabled until either Rosetta 2 supports AVX
+    # instructions in bottles or Homebrew refuses to run under Rosetta 2 (when
+    # ARM support is sufficiently complete):
+    #   https://github.com/Homebrew/homebrew-core/issues/67713
+    #
+    # elsif version >= :big_sur
+    #   :ivybridge
     elsif version >= :mojave
       :nehalem
     else