Add a -fuse-init-array option to cc1 and map to the UseInitArray target

option. On the driver, check if we are using libraries from gcc 4.7 or newer
and if so pass -fuse-init-array to the frontend.
The crtbegin*.o files in gcc 4.7 no longer call the constructors listed in
.ctors, so we have to use .init_array.

llvm-svn: 158694