Auto merge of #112591 - jfgoog:better-dlltool-diagnostics, r=WaffleLapkin

Better diagnostics for dlltool errors.

When dlltool fails, show the full command that was executed. In particular, llvm-dlltool is not very helpful, printing a generic usage message rather than what actually went wrong, so stdout and stderr aren't of much use when troubleshooting.

Author: bors