firefox: Workaround for building on i686-linux

http://hydra.nixos.org/build/13992569

Changed files
+2 -1
pkgs
applications
networking
browsers
firefox
+2 -1
pkgs/applications/networking/browsers/firefox/default.nix
···
]
++ (if debugBuild then [ "--enable-debug" "--enable-profiling"]
else [ "--disable-debug" "--enable-release"
-
"--enable-optimize" "--enable-strip" ])
+
"--enable-optimize${lib.optionalString (stdenv.system == "i686-linux") "=-O1"}"
+
"--enable-strip" ])
++ lib.optional enableOfficialBranding "--enable-official-branding";
enableParallelBuilding = true;