[Buildroot] Update of toolchains for autobuild.b.o

[Thomas Petazzoni]

Hello,

I have just done a round of update in the Buildroot toolchain
configurations used on autobuild.b.o:

 * All pre-built Buildroot toolchains have been rebuilt with Buildroot
   2015.02 (except the SH4 toolchain, since it doesn't build in recent
   Buildroot)

 * Added an ARC external toolchain. The intent here is mainly to have
   an easily available pre-built ARC toolchain.

 * Added a Xtensa external toolchain. Same idea.

 * Added a Buildroot external toolchain built by Buildroot.

 * Added an ARM Musl based toolchain. This is probably the biggest
   change, and will most likely cause a fairly significant number of
   build failures.

 * Changed the ARM internal Buildroot toolchain configuration to use
   uClibc-ng instead of uClibc 0.9.33.

If you see other useful updates to be done in the autobuilder
configurations, please let me know.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com