ghc: add ENV.m32 for 32bit on x64.

- Add `ENV.m32` to fix build errors on 64bit hardware w/ `--32-bit`

Some users want to force 32bit ghc on 64bit hardware.
Ghc, however, comes with several internal libraries like libgmp
that get configured during the build. By forcing `ENV.m32`,
we force all the internal libs to build 32bit also.

Fixes Homebrew/homebrew#15925.
Closes Homebrew/homebrew#15933.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Formula/ghc.rb

@@ -84,6 +84,7 @@ class Ghc < Formula
       if Hardware.is_64_bit? and not build.build_32_bit?
         arch = 'x86_64'
       else
+        ENV.m32 # Need to force this to fix build error on internal libgmp.
         arch = 'i386'
       end