cmake: add WITH_MOLD option for Mold linker support

Add a WITH_MOLD cmake option that auto-configures the build for
the Mold linker. When enabled, cmake finds mold, sets CMAKE_LINKER,
injects -fuse-ld=mold into all linker flag variables, and sets
USING_MOLD_LINKER which gates the Mold-specific workarounds added
in the previous commit.

Usage: cmake -DWITH_MOLD=ON ...

Assisted-by: Claude <claude.ai>
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>