Fix building universal binaries on 32-bit CPUs

[jn: style, update comment]

Closes Homebrew/homebrew#25728.
Fixes Homebrew/homebrew#26834.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/extend/ENV/super.rb

@@ -262,6 +262,14 @@ module Superenv
   def universal_binary
     self['HOMEBREW_ARCHFLAGS'] = Hardware::CPU.universal_archs.as_arch_flags
     append 'HOMEBREW_CCCFG', "u", ''
+
+    # GCC doesn't accept "-march" for a 32-bit CPU with "-arch x86_64"
+    if compiler != :clang && Hardware.is_32_bit?
+      self['HOMEBREW_OPTFLAGS'] = self['HOMEBREW_OPTFLAGS'].sub(
+        /-march=\S*/,
+        "-Xarch_#{Hardware::CPU.arch_32_bit} \\0"
+      )
+    end
   end
 
   def cxx11