cmd/link: add LIBRARY statement only with -buildmode=cshared
When creating a .def file for Windows linking, add a LIBRARY statement
only when building a DLL with -buildmode=cshared. That statement is
documented to instruct the linker to create a DLL, overriding any
other flag that might indicate building an executable.
Fixes #75734
Change-Id: I0231435df70b71a493a39deb639f6328a8e354f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/708815 Reviewed-by: Carlos Amedee <carlos@golang.org> Reviewed-by: Dominic Della Valle <ddvpublic@gmail.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>