Switch host native tests to -Og

This helps to reduce build artifact size significantly, by 1.33GB for
`m cts` in the `out` directory on aosp-main. This should also make them
a lot faster to run.

Also turns off unused-result warning. This analysis is not run on -O0,
leaving behind a ton of code that triggers this warning. Since it is not
an important warning for tests anyway, turn it off by default for tests.

Right now, -Og is just an alias for -O1 in Clang. If the debuggability
is affected due to this change, we can identify and disable the relevant
optimization pass in -Og.

Change-Id: I84383cbc05a53d89c322ecb3679320f1338542e6
Bug: 294796809
Test: presubmit