buildroot

support/config-fragments: add internal defconfigs for glibc and musl

While we have several defconfigs building internal toolchains with
uClibc, we don't have any building internal toolchain with glibc and
musl. However, having such defconfigs is nice when we bump the C
library version, in order to immediately get feedback on build
failures.

Note that while the ARC internal defconfig uses glibc, it uses the
special ARC glibc version, so it doesn't test version bumps of the
upstream glibc C library.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>