Fix potential data race in GC_SysVGetDataStart (SPARC)

Fix potential data race in GC_SysVGetDataStart (SPARC)

* os_dep.c [SVR4 || AUX || DGUX || LINUX && SPARC]
(GC_SysVGetDataStart): Use AO_fetch_and_add(p,0) if available instead
of non-atomic read/write in *p = *p statement (thus, to avoid data race
though it is highly unlikely).