RFR (S) 8231472: Clean up Monitor::_safepoint_check usage to Mutex::_safepoint_check

[coleen.phillimore at oracle.com]

Summary: Change Monitor:: to Mutex:: when allocating a Mutex, and in 
mutexLocker.

Small cleanup.  Tested with tier1 on Oracle platforms.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8231472.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8231472

Thanks,
Coleen