Look into changing gnu optimization setting to -O0 on izumi

[cacraigucar]

On izumi, when using code compiled with the current -Og setting, a good number of variables are "optimized out" when viewing in totalview. Changing this setting in two places in the cmake_macros/gnu.cmake file before running case.build yields variables which are able to be viewed in the totalview debugger.

There was some discussion on investigating the timing impact of making this change on CESM regression tests.

If the timing is not significantly impacted, I would like to suggest that gnu use the -O0 setting for debug cases.

[fischer-ncar]

I just did some quick build tests on izumi using QPC6. -O0 builds look to be faster than -Og.

-Og builds
Total build time: 330.851402 seconds
Total build time: 345.887056 seconds

-O0 builds
Total build time: 284.907677 seconds
Total build time: 275.838053 seconds