gcc-8 and gcc-9 builds using pgo and lto optimization

The recent gcc-8 and gcc-9 uploads to unstable are now built using pgo and lto
optimization.  Not on all architectures, see debian/rules.defs.  On the plus
side the compilers are 7-10% faster, however the build time of the compiler is
much longer, adding 10-20 hours.  If people feel that this isn't worth the extra
build time, please file an issue for the package to disable those optimizations.

Matthias