RFR: 8031126 java/lang/management/ThreadMXBean/ThreadUserTime.java fails intermittently

[Staffan Larsen]

When JDK-8036122 "Fix warning "format not a string literal”” was fixed, the fix introduced a regression in 
os_linux.cpp:slow_thread_cpu_time(). There is a static variable in the method with a format string. The format string was set to different value if NTPL was used. This check was only done once. After the fix, the incorrect format string is used on NPTL for all but the first call.

Since all our linux variants use NPTL now, I suggest a simple fix that has only one format string.

webrev: http://cr.openjdk.java.net/~sla/8031126/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8031126

Thanks,
/Staffan