toolchain-external: bump musl toolchain to 1.1.12

toolchain-external: bump musl toolchain to 1.1.12

While the prebuilt musl toolchains provided by http://musl.codu.org/
had not been updated in a while, a new release based on musl 1.1.12
has been put online in December 2015. This commit updates our external
toolchain package to use this new pre-built toolchain.

Compared to the previous 1.1.6 toolchain, there are some changes:

 - The MIPS big endian soft-float variant is no longer available.
 - The Microblaze variant is no longer available.
 - SuperH 4, both little and big endian, variants have been added.
 - The components have been updated: gcc 5.3 is used, binutils 2.25.1,
   and of course musl 1.1.12.

Besides the update itself, in this commit, we are:

 - Making the musl toolchain non-selectable on MIPS big endian
   soft-float.

 - Making the musl toolchain actually work on MIPS little endian
   soft-float, by downloading the right tarball and setting up the
   right symbolic link.

 - Removing support for the Microblaze variant, and adding support for
   the SH4 variants.

All variants except armeb have been boot tested under Qemu, up to a
Busybox shell prompt. armeb has not been tested due to the lack of a
Qemu configuration for this architecture.

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