[PECOFF] Handle large objects having more than 32768 sections.

The COFF spec says that the SectionNumber field in the symbol table is 16 bit
signed type, but MSVC treats the field as if it is unsigned.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@203901 91177308-0d34-0410-b5e6-96231b3b80d8