toolchain: enable microblaze toolchain

This commit allows to build an internal toolchain for the Microblaze
architecture, with either glibc or eglibc.

Note that we add an explicit list of architectures that are supported
by uClibc, and Microblaze is not part of them, because it currently
doesn't build for this architecture.

[Thomas: add better commit log, add architecture dependencies on
uClibc, to avoid selecting uClibc on Microblaze]

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