toolchain.eclass: pass CONFIG_SHELL to emake, bug #664486

toolchain.eclass: pass CONFIG_SHELL to emake, bug #664486

Christian reports 'make install' failure caused by limitation
of ancient autotools in detecting CONFIG_SHELL:

```
    /bin/bash .../gcc-3.3.6/libstdc++-v3/../mkinstalldirs \
              .../gcc-3.3.6-r2/image//usr/lib/../lib32
    /bin/bash ../libtool  --mode=install /bin/bash .../gcc-3.3.6/install-sh \
        -c libsupc++.la .../gcc-3.3.6-r2/image//usr/lib/../lib32/libsupc++.la
    libtool: install: `.../gcc-3.3.6-r2/image//usr/lib/../lib32/libsupc++.la' \
        is not a directory
```

We already pass CONFIG_SHELL to top-level ./configure.
The workaround is to pass CONFIG_SHELL variable to 'make' as well.

Reported-by: Christian Schmidt
Fixed-by: Christian Schmidt
Bug: https://bugs.gentoo.org/664486
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>