glibc/eglibc: remove reference to experimental from help text

Since 924b8739 we no longer declare glibc and eglibc support as
experimental, so it shouldn't be mentioned in the help text anymore.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>