RFR: 8230877: Rename THREAD_LOCAL_DECL to thread_local

[Per Liden]

ZGC uses thread locals in a number of places. It's currently using using 
the gcc specific __thread, but we'd like to make that compiler agnostic. 
We currently have the THREAD_LOCAL_DECL macro, which does what we want. 
However, I propose that we rename THREAD_LOCAL_DECL to thread_local, for 
the following reasons:

* In C+11, thread_local was standardized. When we upgrade to C++1X, we 
can just remove our own thread_local macros, without the need to touch 
any other code.

* The thread_local keyword is recognized by IDEs/editors, so gets 
correctly highlighted, etc.

* THREAD_LOCAL_DECL looks a bit clunky in declarations (IMHO).

About the patch:

* USE_LIBRARY_BASED_TLS_ONLY now only controls how Thread::current() is 
implemented, not whether thread_local should be defined or not.

* We define thread_local on gcc/clang/solstudio, if we're using a 
pre-C++11 compiler.

* We don't define thread_local on xlc, since thread_local will never be 
used anyway because it currently always defines 
USE_LIBRARY_BASED_TLS_ONLY to 1.

* We don't define thread_local on VS2017, since it's already supported 
with that compiler.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230877
Webrev: http://cr.openjdk.java.net/~pliden/8230877/webrev.0

Testing: Builds on all Oracle supported platforms

/Per