linux: Update to 3.12.25

(cherry picked from commit 0852d9e3643458ebd435b366bb3ecd79b0f47400)
nss: Update to 3.16.3

(cherry picked from commit ea0013a0d9d5a6e7b5cda8a1d6f4dc7c0b20df6e)
firefox: Update to 31.0

Also boldly re-enable parallel building.

(cherry picked from commit 21c7ed54e3617d5561d5ff4c72a8c7505fdce060)
Remove unused function

(cherry picked from commit ee3db692e67dc81e58b31b6fec410dd85fd4efde)
firefox: Build without xulrunner

Fixes #2950.

(cherry picked from commit 5c5f11560329f41573c862b2146f6f7bc5e19d1b)

Conflicts:
	pkgs/applications/networking/browsers/firefox/default.nix
	pkgs/top-level/all-packages.nix
	pkgs/top-level/release.nix
pkgsi686Linux: Use 32b kernels (x86_64 kernels can't be built with 32b gcc).

(cherry picked from commit bde992bbbc7148255d439cbb8b54a772b81e89eb)
gnu: Fix (evaluation of the) call to forceSystem.

(cherry picked from commit 559f423417ea98a50ad0b96a48f23501f80141a6)