CPU: add ability to check for Rosetta

When running within an Intel terminal, `uname -m` and friends return Intel-based
values for compatibility. An Intel shell will also prefer to launch Intel slices of
programs unless the program is ARM-only.

It's an open question how Homebrew should manage running in Intel mode. Should it
continue to behave as though the Mac is Intel-based, like it does now? Should it
recognize it's ARM-based? Either way, it's useful for us to be able to tell whether
the Mac is running under Rosetta or whether it's a real Intel Mac.

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

@@ -63,6 +63,14 @@ module Hardware
         [arch_64_bit, arch_32_bit].extend ArchitectureListExtension
       end
 
+      # True when running under an Intel-based shell via Rosetta on an
+      # Apple Silicon Mac. This can be detected via seeing if there's a
+      # conflict between what `uname` report and the underlying `sysctl` flags,
+      # since the `sysctl` flags don't change behaviour under Rosetta.
+      def running_under_rosetta?
+        intel? && physical_cpu_arm64?
+      end
+
       def features
         @features ||= sysctl_n(
           "machdep.cpu.features",
@@ -109,6 +117,12 @@ module Hardware
 
       private
 
+      # Note: this is more reliable than checking uname.
+      # `sysctl` returns the right answer even when running in Rosetta.
+      def physical_cpu_arm64?
+        sysctl_bool("hw.optional.arm64")
+      end
+
       def sysctl_bool(key)
         sysctl_int(key) == 1
       end

Library/Homebrew/hardware.rb

@@ -154,6 +154,10 @@ module Hardware
 
         "-march=#{arch}"
       end
+
+      def running_under_rosetta?
+        false
+      end
     end
   end