glibc: add 2.19 as a supported version

glibc 2.19 has been released recently
(https://sourceware.org/ml/libc-alpha/2014-02/msg00224.html

). This
commit allows to build a toolchain with this new version. In order to
allow this, we add a version selection that did not exist for
glibc. We default to 2.18, which was the only supported version until
now, and add an option for 2.19.

For microblaze, which uses a specific glibc version, the version
selection choice is not displayed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>