[Qt] GC should be parallel on Qt platform

[Qt] GC should be parallel on Qt platform
https://bugs.webkit.org/show_bug.cgi?id=73309

Patch by Roland Takacs <takacs.roland@stud.u-szeged.hu> on 2012-01-26
Reviewed by Zoltan Herczeg.

These changes made the parallel gc feature available for Qt port.
The implementation of "registerGCThread" and "isMainThreadOrGCThread"
is moved from MainThreadMac.mm to the common MainThread.cpp to make
them available for other platforms.

Measurement results:
V8           speed-up:  1.071x as fast  [From: 746.1ms  To: 696.4ms ]
WindScorpion speed-up:  1.082x as fast  [From: 3490.4ms To: 3226.7ms]
V8 Splay     speed-up:  1.158x as fast  [From: 145.8ms  To: 125.9ms ]

Tested on Intel(R) Core(TM) i5-2320 CPU @ 3.00GHz with 4-core.

* wtf/MainThread.cpp:
(WTF):
(WTF::registerGCThread):
(WTF::isMainThreadOrGCThread):
* wtf/Platform.h:
* wtf/mac/MainThreadMac.mm:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105982 268f45cc-cd09-0410-ab3c-d52691b4dbfc