llvm-readobj: add support for PE32+ (Windows 64 bit executable).

PE32+ supports 64 bit address space, but the file format remains 32 bit.
So its file format is pretty similar to PE32 (32 bit executable). The
differences compared to PE32 are (1) the lack of "BaseOfData" field and
(2) some of its data members are 64 bit.

In this patch, I added a new member function to get a PE32+ Header object to
COFFObjectFile class and made llvm-readobj to use it.