[PECOFF] Fix exported symbol in the import library

There are two ways to specify a symbol to be exported in the module
definition file.

1) EXPORT <external name> = <symbol>
2) EXPORT <symbol>

In (1), you give both external name and internal name. In that case,
the linker tries to find a symbol using the internal name, and write
that address to the export table with the external name. Thus, from
the outer world, the symbol seems to be exported as the external name.

In (2), internal name is basically the same as the external name
with an exception: if you give an undecorated symbol to the EXPORT
directive, and if the linker finds a decorated symbol, the external
name for the symbol will become the decorated symbol.

LLD didn't implement that exception correctly. This patch fixes that.