oldest_cpu: Bottling architecture of ARM is armv6

armv6 is the oldest supported 32-bit ARM architecture
armv8-a is the oldest 64-bit ARM architecture.

See https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html
and https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html

Library/Homebrew/hardware.rb

@@ -9,6 +9,8 @@ class << self
       OPTIMIZATION_FLAGS = {
         core2: "-march=core2",
         core: "-march=prescott",
+        armv6: "-march=armv6",
+        armv8: "-march=armv8-a",
         dunno: "-march=native",
       }.freeze
 
@@ -147,6 +149,12 @@ def self.oldest_cpu
       else
         :core
       end
+    elsif Hardware::CPU.arm?
+      if Hardware::CPU.is_64_bit?
+        :armv8
+      else
+        :armv6
+      end
     else
       Hardware::CPU.family
     end