Object/COFF: Support large relocation table.

NumberOfRelocations field in COFF section table is only 16-bit wide. If an
object has more than 65535 relocations, the number of relocations is stored
to VirtualAddress field in the first relocation field, and a special flag
(IMAGE_SCN_LNK_NRELOC_OVFL) is set to Characteristics field.

In test we cheated a bit. I made up a test file so that it has
IMAGE_SCN_LNK_NRELOC_OVFL flag but the number of relocations is much smaller
than 65535. This is to avoid checking in a large test file just to test a
file with many relocations.

Differential Revision: http://llvm-reviews.chandlerc.com/D3139

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204418 91177308-0d34-0410-b5e6-96231b3b80d8