cmd/compile: improve output when TestAssembly build fails
cmd/compile: improve output when TestAssembly build fails

Change-Id: Ibee84399d81463d3e7d5319626bb0d6b60b86bd9
Reviewed-on: https://go-review.googlesource.com/40861
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,Cofyc/go,christopher-henderson/Go,cloudflare/go,vsekhar/elastic-go,cloudflare/go,vsekhar/elastic-go,akutz/go,codestation/go,codestation/go,codestation/go,ImJasonH/go,nawawi/go,codestation/go,ImJasonH/go,akutz/go,vsekhar/elastic-go,polyverse-security/go,polyverse-security/go,nawawi/go,nawawi/go,derekmarcotte/go,Cofyc/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,derekmarcotte/go,josharian/go,Cofyc/go,vsekhar/elastic-go,CAFxX/go,vsekhar/elastic-go,akutz/go,ImJasonH/go,nawawi/go,vsekhar/elastic-go,CAFxX/go,cloudflare/go,josharian/go,polyverse-security/go,josharian/go,ImJasonH/go,polyverse-security/go,golang/go,CAFxX/go,codestation/go,golang/go,nawawi/go,ImJasonH/go,golang/go,nawawi/go,derekmarcotte/go,derekmarcotte/go,Cofyc/go,ImJasonH/go,Cofyc/go,polyverse-security/go,cloudflare/go,vsekhar/elastic-go,vsekhar/elastic-go,derekmarcotte/go,nawawi/go,josharian/go,cloudflare/go,codestation/go,Cofyc/go,nawawi/go,derekmarcotte/go,codestation/go,josharian/go,akutz/go,Cofyc/go,polyverse-security/go,CAFxX/go,akutz/go,CAFxX/go,golang/go,josharian/go,ImJasonH/go,CAFxX/go,CAFxX/go,golang/go,akutz/go,cloudflare/go,golang/go,Cofyc/go,polyverse-security/go,ImJasonH/go,golang/go,codestation/go,derekmarcotte/go,akutz/go,josharian/go,polyverse-security/go,derekmarcotte/go