RFD: test requirements for slow platforms

[Joern Rennecke]

We generally require bootstraps for patches to native-capable targets.
This is quite time consuming for targets like rs6000 or ia64 where
the available machines in the compile farm are have low processing speed
and/or memory, and for rs6000 also suffer issues with mpc / gmp / mpfr  
libraries
and 32/64 bit configurations.

When risks of the patch mostly involve type checking or things that could be
caught with a simple compilation, could we relax this testing requirement
to do a cross-build of all-gcc all-target-libgcc with a recent fully
bootstrapped compiler, with -Werror enabled.
We probably then want to make --enable-werror to work for cross-configurations
like stage2/3 in a bootstrap.