cmd/go: display program name when reporting crash

Fix by atom (from CL 89190044), comment and test by me.

Fixes #6823.

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=0xe2.0x9a.0x9b, adg, golang-codereviews, iant, r
https://golang.org/cl/148180043

src/cmd/go/build.go

@@ -1469,6 +1469,14 @@ func (b *builder) runOut(dir string, desc string, env []string, cmdargs ...inter
 			continue
 		}
 
+		// err can be something like 'exit status 1'.
+		// Add information about what program was running.
+		// Note that if buf.Bytes() is non-empty, the caller usually
+		// shows buf.Bytes() and does not print err at all, so the
+		// prefix here does not make most output any more verbose.
+		if err != nil {
+			err = errors.New(cmdline[0] + ": " + err.Error())
+		}
 		return buf.Bytes(), err
 	}
 }

src/cmd/go/test.bash

@@ -60,6 +60,17 @@ if ! grep -q "^$fn:" $d/err.out; then
 fi
 rm -r $d
 
+TEST 'program name in crash messages'
+linker=$(./testgo env GOCHAR)l
+d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
+./testgo build -ldflags -crash_for_testing $(./testgo env GOROOT)/test/helloworld.go 2>$d/err.out || true
+if ! grep -q "/tool/.*/$linker" $d/err.out; then
+	echo "missing linker name in error message"
+	cat $d/err.out
+	ok=false
+fi
+rm -r $d
+
 # Test local (./) imports.
 testlocal() {
 	local="$1"

src/cmd/ld/pobj.c

@@ -45,6 +45,8 @@ char*	paramspace	= "FP";
 void
 main(int argc, char *argv[])
 {
+	int i;
+
 	linkarchinit();
 	ctxt = linknew(thelinkarch);
 	ctxt->thechar = thechar;
@@ -64,6 +66,13 @@ main(int argc, char *argv[])
 	INITENTRY = 0;
 	linkmode = LinkAuto;
 
+	// For testing behavior of go command when tools crash.
+	// Undocumented, not in standard flag parser to avoid
+	// exposing in usage message.
+	for(i=1; i<argc; i++)
+		if(strcmp(argv[i], "-crash_for_testing") == 0)
+			*(volatile int*)0 = 0;
+
 	if(thechar == '5' && ctxt->goarm == 5)
 		debug['F'] = 1;