Remove WTF::SpinLock

Remove WTF::SpinLock
https://bugs.webkit.org/show_bug.cgi?id=148208

Reviewed by Geoffrey Garen.

Source/JavaScriptCore:

Remove the one remaining use of SpinLock.

* API/JSValue.mm:
(handerForStructTag):

Source/WTF:

Remove the SpinLock.h file and remove references to the SpinLock class. Put the old SpinLock
algorithm in LockSpeedTest.cpp - which isn't compiled as part of a WTF or WebKit build - just
so we can still benchmark our locking algorithms against a spinlock baseline.

* WTF.vcxproj/WTF.vcxproj:
* WTF.xcodeproj/project.pbxproj:
* benchmarks/LockSpeedTest.cpp:
* wtf/CMakeLists.txt:
* wtf/Lock.h:
* wtf/SpinLock.h: Removed.
* wtf/WordLock.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@188677 268f45cc-cd09-0410-ab3c-d52691b4dbfc