Use LLVM as a backend for the fourth-tier DFG JIT (a.k.a. the FTL JIT)

Use LLVM as a backend for the fourth-tier DFG JIT (a.k.a. the FTL JIT)
https://bugs.webkit.org/show_bug.cgi?id=112840

Rubber stamped by Geoffrey Garen.

Source/JavaScriptCore:
* Configurations/FeatureDefines.xcconfig:

Source/WebCore:
It already has a lot of tests.

* Configurations/FeatureDefines.xcconfig:

Source/WebKit/mac:
* Configurations/FeatureDefines.xcconfig:

Source/WebKit2:
* Configurations/FeatureDefines.xcconfig:

Tools:
* Scripts/build-jsc: Enable it on Mac.
* Scripts/build-webkit: Enable it on Mac.
* Scripts/export-llvm-build: Make it slightly easier to export the build if you've done a "make install".

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167958 268f45cc-cd09-0410-ab3c-d52691b4dbfc