build: add Mold linker compatibility

build: add Mold linker compatibility

Mold handles several linker features differently from ld.bfd:

- --exclude-libs,ALL hides .symver-aliased symbols (e.g. rados_*)
  even when the version script lists them as global. Detect Mold
  (via USING_MOLD_LINKER) and disable --exclude-libs; version
  scripts already control symbol visibility.

- Duplicate symbols across static archives are treated as errors.
  Merge crimson-alien-common into crimson-alienstore as a single
  archive. For crimson-osd, link crimson-alienstore before
  crimson-common with --allow-multiple-definition so the alien
  thread's non-crimson definitions win (Mold picks first definition).

- Python/Cython extension builds (via Distutils.cmake) invoked the
  system default linker. Pass -fuse-ld= via MOLD_FUSE_LD_FLAG so
  extensions link with the configured linker.

- Add rados_* and _rados_* to librados.map global section for Mold
  compatibility with .symver aliases.

Co-authored-by: Mark Kogan <mkogan@redhat.com>
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>