bindexplib: Always export executable symbols, even they are also readable

Previously bindexplib discarded read-only non-function symbols even in
executable/code sections, but in some specific cases they could still mark
functions.

An example is provided by nop.asm in the AuoExportDll test, which exports
a function only marked by a label. This symbol can be used from C/C++
code, but without this change it would result in an unresolved external
symbol when built as a DLL on Windows.