RFR: 8140348: Convert TraceSafepoint to Unified Logging

[Rachel Protacio]

Hello,

Please review my next logging enhancement.

Summary: The former -XX:+TraceSafepoint flag is updated to the unified 
logging framework and is now replaced with -Xlog:safepoint in product mode.

open webrev: http://cr.openjdk.java.net/~rprotacio/8140348/
bug link: https://bugs.openjdk.java.net/browse/JDK-8140348
testing: Passes JPRT, RBT, and RefWorkload performance testing.

A compatibility request has been accepted with regard to this change.

Thank you,
Rachel