BUILD: makefile: never force -latomic, set USE_LIBATOMIC instead

BUILD: makefile: never force -latomic, set USE_LIBATOMIC instead

Two places, 51Dv4 and AIX7.2, used to forcefully add -latomic to the
ldflags (and via different variables). This must not be done because
it depends on compiler, arch, etc. USE_LIBATOMIC=implicit is much
better: it allows the user to forcefully disable it if undesired.
The LIBATOMIC_LDFLAGS are set to -latomic and automatically added
to OPTIONS_LDFLAGS.

It will make this dependency appear in haproxy -vv but that's not
and issue and it may even sometimes help when troubleshooting.