It's not included in the Linux distributions of FPC/Lazarus for the same reason that those distributions don't include gdb, as or ld: they rely on you installing the OS-versions of those tools if you need them/want to use them.
windres is available on Ubuntu 16 in e.g. the package binutils-mingw-w64-i686, as i686-w64-mingw32-windres. In general, it should be available in any distribution as part part of their win32/win64 cross binutils package.