Switch from RelWithDebInfo to MinSizeRel + -g for Linux amd64

[janisozaur]

In #9357 it was found that
the archive to be uploaded is too big, over 32MiB quota imposed by
Google App Engine. To alleviate that, switch GCC to -Os -g to help
reduce the code size a bit.

Fixes #9357

[janisozaur]

Let's see how big the archive is.

Ping @JarnoVgr

[janisozaur]

I have some other ideas how to further improve that:

1. Use system gtest. While there is a warning, I think I know what I'm doing™ and it should be fine.

• This should have the benefit of exposing cmake's gtest_discover_tests

2. Group the tests into common binary. Multiple smaller binaries are usually quicker to build than large one and easier to start porting, but I'm willing to sacrifice that for the sake of being able to produce binaries we distribute

[janisozaur]

Any of the gtest changes won't help, as tests are not being packaged.

[janisozaur]

Resulting package is 31752767 bytes.

33554432 (32MiB) - 31752767 = 1801665 ≃ 1760kiB