uclibc: add a special Xtensa version

Currently, the Xtensa architecture uses the "snapshot" version of
uClibc. This means that the build is not reproducible, since it will
pick whatever latest version of uClibc is available at the moment of
the build.

This commit replaces that by adding a special Xtensa version, which
points to a well-known Git commit. This is something we should
hopefully be able to remove once the uClibc people realize that doing
a 0.9.34 release would be useful.

Should probably fix:

  http://autobuild.buildroot.org/results/5d6/5d6072a038acf894d832704e36c1d43f0254abf5/build-end.log



at least I wasn't able to reproduce the build problem.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[baruch: use a more recent uClibc version]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>