[PECOFF] Avoid using statically initialized std::vector.

Also slightly reduces PECOFFLinkingContext's memory footprint (~128B).

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194787 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Minimum size of DOS stub is 64 byte.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194791 91177308-0d34-0410-b5e6-96231b3b80d8