Add support for Mac OS X 64bit builds with GYP

Add support for Mac OS X 64bit builds with GYP

Note that in order to build for 64bits mode, you'll have
to specify the target architecture explicitely, the default
is still 32bits for Mac OS X.

Example with make and gcc:

$ export GYP_GENERATORS=make
$ make dependencies
$ make -j 8 library=shared x64.release

Example with make and clang:

$ export GYP_GENERATORS=make
$ export CC=/usr/bin/clang
$ export CXX=/usr/bin/clang++
$ export GYP_DEFINES="clang=1"
$ make dependencies
$ make -j 8 library=shared x64.release

Example with xcode:

$ export GYP_GENERATORS=xcode
$ build/gyp_v8 -Dtarget_arch=x64
$ xcodebuild -project build/all.xcodeproj -configuration Release

Contributed by Filipe David Manana <fdmanana@gmail.com>

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/9808065

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11199 ce2b1a6d-e550-0410-aec6-3dcde31c8c00