toolchain/buildroot: default to glibc where possible/convenient

Default to glibc over eglibc where it's possible and/or convenient.
Since the eglibc project is basically gone and merged with glibc it
doesn't make sense to keep defaulting to it for architectures that
aren't uClibc-capable.

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