RFR (S): 8149901: [Solaris] Use of -XX:+UseThreadPriorities crashes fastdebug

[David Holmes]

bug: https://bugs.openjdk.java.net/browse/JDK-8149901

webrev: http://cr.openjdk.java.net/~dholmes/8149901/webrev/

JDK-8038473 removed the old code pertaining to use of the T1 threads 
library and related unused flags - like DefaultThreadPriority. One chunk 
of code it removed from os::create_thread was:

// Set the default thread priority. If using bound threads, setting
// lwp priority will be delayed until thread start.
set_native_priority(thread, DefaultThreadPriority == -1 ?
                               java_to_os_priority[NormPriority] :
                               DefaultThreadPriority);

but by removing this, the logic in thread_native_entry (formerly 
java_start) would read back an uninitialized field from the OSThread 
instance for any thread type (eg VMThread) which did not explicitly set 
the priority before calling os::start_thread - and that would cause a 
range check assertion to fire.

The simple fix, because we only deal with bound threads (and so we know 
we will skip most of os::set_native_priority) is to just push a default 
priority into the OSThread instance directly:

osthread->set_native_priority(NormPriority);

Ideally I would have fixed this in the OSThread constructor but it is 
shared code and knows nothing about the native_priority field on Solaris.

I found a number of related issues when looking closely at this code, 
and have filed:

https://bugs.openjdk.java.net/browse/JDK-8157010

as a followup cleanup for Java 10.

Thanks,
David