complibs: introduce a two-stage select

Directly select-ing the required companion libraries means we can not
disable them. That's OK for now, as we systematically build them when
they are required.

But with distros coming up-to-speed, we will need to disable the build
later-on.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="EXPERIMENTAL MIPS-64 n32 toolchain."