Fix windows build with golang 1.15

With golang 1.15.5, attempting the windows cross build fails with mingw
ld reporting "Error: export ordinal too large: 69634", which is:

golang/go#40795

The suggested workaround for executables is -buildmode=exe.

Upstream-repository: operator-registry
Upstream-commit: 116a5cc4355c1fb905c72a5f049c581f7e671623

Author: yselkowitz

staging/operator-registry/Makefile

@@ -33,7 +33,7 @@ cross: opm_version_flags=-ldflags "-X '$(PKG)/cmd/opm/version.gitCommit=$(GIT_CO
 cross:
 ifeq ($(shell go env GOARCH),amd64)
 	GOOS=darwin CC=o64-clang CXX=o64-clang++ CGO_ENABLED=1 $(GO) build $(opm_version_flags) $(TAGS) -o "bin/darwin-amd64-opm" --ldflags "-extld=o64-clang" ./cmd/opm
-	GOOS=windows CC=x86_64-w64-mingw32-gcc CXX=x86_64-w64-mingw32-g++ CGO_ENABLED=1 $(GO) build $(opm_version_flags) $(TAGS)  -o "bin/windows-amd64-opm" --ldflags "-extld=x86_64-w64-mingw32-gcc" ./cmd/opm
+	GOOS=windows CC=x86_64-w64-mingw32-gcc CXX=x86_64-w64-mingw32-g++ CGO_ENABLED=1 $(GO) build $(opm_version_flags) $(TAGS)  -o "bin/windows-amd64-opm" --ldflags "-extld=x86_64-w64-mingw32-gcc" -buildmode=exe ./cmd/opm
 endif
 
 .PHONY: static