superenv: Allow ENV.m32 (for 32bit builds)

Superenv normally filters out "-m32" flag, preventing 32bit builds.
Some software, however, still only work in 32bit mode.

If ENV.m32 is called, superenv does not filter out the "-m32" flag.
Also note, superenv, does not explicitly add the -m32 flag and
expects the build system of the software to know when and where to
provide this flag.

Closes Homebrew/homebrew#16350.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/ENV/4.3/cc

@@ -101,8 +101,11 @@ class Cmd
       case arg = whittler.next
       when '-arch', /^-Xarch_/
         whittler.next
+      when '-m32'
+        # If ENV.m32 was set, we allow the "-m32" flag, but we don't add anything
+        args << '-m32' if cccfg? '3'
       when /^-g\d?/, /^-gstabs\d+/, '-gstabs+', /^-ggdb\d?/, '-gdwarf-2',
-           /^-march=.+/, /^-mtune=.+/, '-m64', '-m32',
+           /^-march=.+/, /^-mtune=.+/, '-m64',
            /^-O[0-9zs]?$/, '-fast',
            '-pedantic', '-pedantic-errors'
       when '-fopenmp', '-lgomp'

Library/Homebrew/superenv.rb

@@ -68,6 +68,11 @@ class << ENV
     append 'HOMEBREW_CCCFG', "u", ''
   end
 
+  # m32 on superenv does not add any flags. It prevents "-m32" from being erased.
+  def m32
+    append 'HOMEBREW_CCCFG', "3", ''
+  end
+
   private
 
   def determine_cc
@@ -205,7 +210,7 @@ class << ENV
 
 ### NO LONGER NECESSARY OR NO LONGER SUPPORTED
   def noop(*args); end
-  %w[m64 m32 gcc_4_0_1 fast O4 O3 O2 Os Og O1 libxml2 minimal_optimization
+  %w[m64 gcc_4_0_1 fast O4 O3 O2 Os Og O1 libxml2 minimal_optimization
     no_optimization enable_warnings x11
     set_cpu_flags
     macosxsdk remove_macosxsdk].each{|s| alias_method s, :noop }