cmake: add "-Og" to CMAKE_C_FLAGS_DEBUG

cmake: add "-Og" to CMAKE_C_FLAGS_DEBUG

without specifying -Og, -O0 is used. as per
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

> -O0
>  Reduce compilation time and make debugging produce the expected
>  results. This is the default.

and

> -Og
>  Optimize debugging experience. -Og should be the optimization level of
>  choice for the standard edit-compile-debug cycle, offering a reasonable
>  level of optimization while maintaining fast compilation and a good
>  debugging experience. It is a better choice than -O0 for producing
>  debuggable code because some compiler passes that collect debug
>  information are disabled at -O0.

and Debug is the default built type if .git directory is found under
the root source directory, so by adding "-Og -g" to CMAKE_C_FLAGS_DEBUG,
developers can have better debugging experience when testing Ceph
built from a git repo. but the downside is that it might take longer
to build the tree.

this change should also enable us to link crimson on aarch64.

the same applies to Clang,

> -O0 Means “no optimization”: this level compiles the fastest and generates the most debuggable code.
> -O1 Somewhere between -O0 and -O2.
> -O2 Moderate level of optimization which enables most optimizations.
> -Og Like -O1. In future versions, this option might disable different optimizations in order to improve debuggability.

see
https://clang.llvm.org/docs/CommandGuide/clang.html#cmdoption-o0

see also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101448

Fixes: https://tracker.ceph.com/issues/51441
Signed-off-by: Kefu Chai <kchai@redhat.com>