tree-wide: platforms = platforms.gnu; -> platforms = gnu ++ linux

"platforms.gnu" has been linux-only since at least 17.03:

$ nix eval -f channel:nixos-17.03 lib.platforms.gnu
[ "i686-linux" "x86_64-linux" "armv5tel-linux" "armv6l-linux" "armv7l-linux" "aarch64-linux" "mips64el-linux" ]

Unlike platforms.linux, platforms.gnu indicates "must use glibc"
which for the most part is not intended.

Replacing platforms.gnu with platforms.linux would be the same "today"
but let's err on preserving existing behavior and be optimistic
about platforms these packages work on.