firefox: Workaround for building on i686-linux

http://hydra.nixos.org/build/13992569
(cherry picked from commit 4ead67b785fc666aaebaa3958b034dca28d61f19)
linux: Update to 3.12.28

(cherry picked from commit 19b1fafe5fac5eccb426b8d9854db3ac93a448f7)