tool/gocross: make all Windows DLLs build with static libgcc

In this commit, we have updated the build process for our Windows DLLs
to link statically with libgcc, ensuring our Windows DLLs are self-contained.

Updates tailscale#10617

Signed-off-by: Nick Khyl <nickk@tailscale.com>

tool/gocross/autoflags.go

@@ -96,6 +96,7 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		cgo = true
 		buildFlags = append(buildFlags, "-buildmode=c-shared")
 		ldflags = append(ldflags, "-H", "windows", "-s")
+		cgoLdflags = append(cgoLdflags, "-static")
 		var mingwArch string
 		switch targetArch {
 		case "amd64":