[PECOFF] Create .edata section for the DLL export table.

This is the first patch to emit data for the DLL export table. The DLL export
table is the data strucutre used by the Windows loader to find the address of
exported function from DLL. With this patch, LLD is able to emit a valid DLL
export table which the Windows loader can interpret and load.

DLL support is not complete yet; the linker needs to emit an import library
for a DLL, otherwise the linker cannot link against the DLL. We also do not
support export-only-by-ordinal yet.