gcc-final: disable shared build for static

Disable shared build for host-gcc-final when building for static targets.
We really want static or shared, there's no such thing as "preferring static"
since we can't choose with any degree of granularity for which packages.
And it confuses linking scripts having both available at the same time. Fixes:
http://autobuild.buildroot.net/results/c54/c54bdf88eff6d60c7001cb0e2cb6792cc75178db/



[Thomas: slightly amend the commit to factorize the installation of
static libraries.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>