external-toolchain: mention MIPS and PowerPC CodeSourcery toolchains

#

#  * Toolchains generated by Crosstool-NG

#  * Toolchains generated by Buildroot

#  * ARM toolchains made available by Codesourcery

#  * ARM, MIPS and PowerPC toolchains made available by

#    Codesourcery. For the MIPS toolchain, the -muclibc variant isn't

#    supported yet, only the default glibc-based variant is.

#

# The basic principle is the following

#