Switching to GCC 4.8

[Ludovic Courtès]

So we want to switch to GCC 4.8 as the default compiler.  I tried that
in ‘core-updates’, but ‘gcc-cross-boot0’ fails to build because some of
its helper programs built natively (‘genchecksum’, ‘gcc-nm’, etc.) rely
on C++ headers not provided by our current bootstrap GCC.  Some can be
easily worked around (e.g., replace <cstring> by <string.h>), but others
cannot (e.g., <iosfwd>.)

Thus, we’ll have to update the bootstrap GCC.

The good thing is that it will allow us to have same-version binaries
for x86_64/i686 and mips64el.  Perhaps we can achieve fixed-point while
we’re at it.

Ludo’.