[PECOFF] Fix __ImageBase symbol.

__ImageBase is an absolute symbol whose address is the same as the image base
address. What we did before this patch was to create __ImageBase symbol as a
symbol whose *contents* (not location) is the image base address, which is
clearly wrong.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193565 91177308-0d34-0410-b5e6-96231b3b80d8