lib: drop (replace) debug_build()

lib: drop (replace) debug_build()

Its expansion shouldn't be tied to NDEBUG - down the road we may want to
allow enabling assertions independently of CONFIG_DEBUG. Replace the few
uses by a new xen_build_info() helper, subsuming gcov_string at the same
time (while replacing the stale CONFIG_GCOV used there) and also adding
CONFIG_UBSAN indication.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>