cmake: Add GCC -Og flag fallback to -O0.

The -Og (optimise for debugging) flag is only available for GCC 4.8.0
and above, and specifying it for a GCC version lower than 4.8.0 will
result in a compilation error.

This commit adds a check for compiler -Og optimisation flag support and
a fallback to -O0 (disable optimisation) when -Og flag is unsupported.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>