PECOFF: make dumpbin tool happy.

The dumpbin tool in the MSVC toolchain cannot handle an executable created
by LLD if the executable contains a long section name.

In PE/COFF, a section name is stored to a section table entry. Because the
section name field in the table is only 8 byte long, a name longer than
that is stored to the string table and the offset in the string table is
stored to the section table entry instead.

In order to look up a string from the string table, tools need to handle
the symbol table, because the string table is defined as it immediately
follows the symbol table.

And seems the dumpbin doesn't like zero-length symbol table.

This patch teaches LLD how to emit a dummy symbol table. The dummy table
has one dummy entry in it.

llvm-svn: 228900