[yaml2obj][COFF] Add support for extended relocation tables

Summary:
The tool does not correctly handle COFF sections with extended relocation tables (with IMAGE_SCN_LNK_NRELOC_OVFL bit set), this patch fixes this problem.

But I have cheated a bit in the test (to make it smaller) because extended relocation table is supposed to be used when the number of relocations exceeds 65534. Otherwise the test size would be pretty big.

Reviewers: jhenderson, MaskRay, mstorsjo

Reviewed By: mstorsjo

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70251