build: switch debug mode from -O0 to -Og
build: switch debug mode from -O0 to -Og

-Og is advertised as debug-friendly optimization, both in compile time
and debug experience. It also cuts sstable_mutation_test run time in half:

Changing -O0 to -Og

Before:

real    16m49.441s
user    16m34.641s
sys    0m10.490s

After:

real    8m38.696s
user    8m26.073s
sys    0m10.575s

Message-Id: <01fc9f3672b1fbeaa9f60a5dadc751a885c871a3@scylladb.com>

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