tools: Use -Og for debug builds when available

tools: Use -Og for debug builds when available

The recommended optimisation level for debugging is -Og, and is what tools
such as gdb prefer.  In practice, it equates to -01 with a few specific
optimisations turned off.

abi-dumper in particular wants the libraries it inspects in this form.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>