RFR (S) 8074355: make MutexLocker smarter about non-JavaThreads

[coleen.phillimore at oracle.com]

Summary: Use safepoint_check_always/safepoint_check_never instead of 
safepoint_check_sometimes for locks that are taken by JavaThreads and 
non-JavaThreads

This is a small cleanup to avoid having safepoint_check_sometimes. 
Threads_lock is the only one remaining.  'Sometimes' was used for some 
locks because they were taken out by GC or other threads and 
JavaThreads, where the other threads weren't going to check for the 
safepoint anyway since they don't participate in the safepoint 
protocol.  There could be further cleanups here.  We could make the 
safepoint checking related to the rank of the lock in the future, or the 
safepoint checking could be a property of the lock so we don't have to 
pass in Mutex::_no_safepoint_check_flag (except Threads_lock).  I think 
these could be further enhancements after this.

Tested with hs-tier1-6, runThese jck with G1, CMS, Serial and ParallelGC.

8074355: make MutexLocker smarter about non-JavaThreads
open webrev at http://cr.openjdk.java.net/~coleenp/2019/8074355.01/webrev

Thanks,
Coleen