Object: handle extensions properly in def files

When given an extension as part of the `library` directive in a def
file, the extension is preserved/honoured by link/lib.  Behave similarly
when parsing the def file.  This requires checking if a native extension
is provided as a keyword parameter.  If no extension is present, append
a standard `.dll` or `.exe` extension.

This is best tested via lld, and I will add tests there as a follow up.

llvm-svn: 308383