change dpkg-buildflags to default to -O3, not -O2, on ppc64el

Per Debian policy, dpkg currently defaults to -O2 for optimization flags on all architectures. According to people more familiar with the architecture than I, -O3 is a better default for ppc64el; because while -O2 includes only those optimizations that *always* improve performance, whereas -O3 includes optimizations that will sometimes *decrease* performance, the architecture-specific trade-offs on POWER mean that -O3 is better for the vast majority of real software, and far outweighs any downsides.

So for ppc64/ppc64el, dpkg should default to -O3 in dpkg-buildflags, not -O2.

It may be the case that we should make this same change for other architectures as well, but not before proving that there's a benefit. The change should also be forwarded to Debian, but that will mean discussing first whether and with what evidence to change Debian policy.