8016697: Use stubs to implement safefetch
Summary: Implement Safefetch as stub routines. This reduces compiler and os dependencies.
Reviewed-by: twisti, kvn
8020155: PSR:PERF G1 not collecting old regions when humongous allocations interfer
Summary: Take _last_young_gc into account when deciding on starting a concurrent mark. Also reviewed-by: per.liden@oracle.com.
Reviewed-by: tschatzl, johnc