WL#10343: Switch GCC optimization from -O3 to -O2

We use the GCC that is included in each Linux distro. The exception
is RHEL/OEL, where the GCC is so old that we use Red Hat's “Developer
Toolsets”, which contain newer versions. We have now upgraded from Developer
Toolset 3 (GCC 4.9) to 6 (GCC 6.2), and all new builds (including
our official binaries) on these platforms are done using Toolset 6.

To our knowledge, all Linux distros use optimization switch -O2, and we will use
the same optimization level.

SysQA performance testing has shown 0-6% performance improvement in Sysbench by
switching from -O3 to -O2, likely due to the reduction in binary size with -O2.
With the new Toolset 6, several DBT3 queries show a very significant performance
improavement of up to 50%.