disk cache: Link with -latomic if necessary

The disk cache implementation uses 64-bit atomic operations. For some
architectures, such as 32-bit ARM, GCC will not be able to translate
these operations into atomic, lock-free instructions and will instead
rely on the external atomics library to provide these operations.

Check at configuration time whether or not linking against libatomic
is necessary and if so, create a dependency that can be used while
linking the mesautil library.

This is the meson equivalent of 2ef7f23820a6 ("configure: check if
-latomic is needed for __atomic_*").

For some background information on this, see:

	https://gcc.gnu.org/wiki/Atomic/GCCMM

Changes in v2:
- clarify meaning of lock-free in commit message
- fix build if -latomic is not necessary

Acked-by: Matt Turner <[email protected]>
Reviewed-by: Dylan Baker <[email protected]>
Signed-off-by: Thierry Reding <[email protected]>

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