RFR: 8036702: Refactor os::safe_for_sender to work with non-Java threads

[Erik Helin]

Hi all,

this patch changes the type of the parameter to the function 
os::safe_for_sender from JavaThread* to Thread*. There is no code in 
os::safe_for_sender that uses members that are specific to JavaThread.

This change is in preparation for JDK-8036699, which will do some stack 
walking on non-Java thread and makes use of os::safe_for_sender for 
non-Java threads.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8036702

Webrev:
http://cr.openjdk.java.net/~ehelin/8036702/webrev.00/

Testing:
- JPRT

Thanks,
Erik