manual: customize-toolchain.txt: update internal backend section

Mention all supported C libraries by the internal Buildroot toolchain
backend.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>