buildFHSEnv: fix `NIX_LDFLAGS` propagation to `ld` wrapper

Before the change the following command did not work:

    $ nix develop -i --impure --expr 'with import <nixpkgs> { system = "i686-linux"; }; (buildFHSUserEnv { name = "t"; targetPkgs = ps: with ps; [ libmpc stdenv.cc ]; }).env'
    $ ld -lmpc -o a
    ld: cannot find -lmpc: No such file or directory

It is expected to work as `NIX_LDFLAGS` does contain valid values:

    $ echo $NIX_LDFLAGS
    -L/usr/lib -L/usr/lib32

Note that for `gcc` it does work:

    $ printf "int main(){}" |  gcc -x c - -lmpc -o a

It happens because `HOST` role is enabled for `cc`:

    $ echo $NIX_CC_WRAPPER_TARGET_HOST_i686_unknown_linux_gnu
    1

But not for `BINTOOLS`:

    $ echo $NIX_BINTOOLS_WRAPPER_TARGET_HOST_i686_unknown_linux_gnu
    <empty>

The change adds BINTOOLS role and fixes linking:

    $ nix develop -i --impure --expr 'with import ~/nm { system = "i686-linux"; }; (buildFHSUserEnv { name = "t"; targetPkgs = ps: with ps; [ libmpc stdenv.cc ]; }).env'
    $ printf "int main(){}" |  gcc -x c - -lmpc -o a
    $ ld -lmpc -o a
    ld: warning: cannot find entry symbol _start; not setting start address