mappings: Add libc-malloc-debug for malloc perturbation

Enabling glibc malloc perturbation during CI is a cheap way to catch several malloc usage problems. This used to be easy (alter TESTS_ENVIRONMENT in Makefile.am to include MALLOC_CHECK_=1 and MALLOC_PERTURB_=$(random) for a proper setting of random), but has gotten progressively harder with newer glibc (you now have to LD_PRELOAD a specific library, and per https://issues.redhat.com/browse/RHEL-35971, Fedora 40 intentionally moved libc_malloc_debug.so.0 out of glibc into glibc-utils).

As several lcitool clients (at least nbdkit and libnbd) want to use malloc debugging during CI, it is worth adding a new mapping for this purpose.

Signed-off-by: Eric Blake eblake@redhat.com