toolchain-buildroot: glibc and eglibc support is no longer experimental

The glibc and eglibc support has been introduced since a little bit of
time now, I believe we can remove the "experimental" statement next to
it.

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