[COFF] Add support for automatically exporting all symbols

GNU ld automatically exports all symbols if no symbols have
been chosen to export via either def files or dllexport attributes.
The same behaviour can also be enabled via the GNU ld option
--export-all-symbols, in case some symbols are marked for export
via a def file or dllexport attribute.

The list of excluded symbols is from GNU ld, minus the
cygwin specific symbols.

Also add support for outputting the actual list of exported
symbols in a def file, as in the GNU ld option --output-def.

These options in GNU ld are documented in
https://sourceware.org/binutils/docs/ld/WIN32.html.

This currently exports all symbols from object files pulled in
from libmingw32 and libmingwex and other static libraries
that are linked in.

Differential Revision: https://reviews.llvm.org/D38760

llvm-svn: 315562