cmd/dist, cmd/go: pass -fmessage-length=0 when using clang

That option turns off word wrapping of individual
error messages generated by clang. The wrapping
makes the errors harder to read and conflicts with the
idea of a terminal window that can be resized.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/35810043

src/cmd/dist/build.c

@@ -677,6 +677,8 @@ install(char *dir)
 			vadd(&gccargs, "-fno-caret-diagnostics");
 			// clang is too smart about unused command-line arguments
 			vadd(&gccargs, "-Qunused-arguments");
+			// disable line wrapping in error messages
+			vadd(&gccargs, "-fmessage-length=0");
 		}
 		if(streq(gohostos, "darwin")) {
 			// golang.org/issue/5261

src/cmd/go/build.go

@@ -1890,6 +1890,8 @@ func (b *builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 		a = append(a, "-fno-caret-diagnostics")
 		// clang is too smart about command-line arguments
 		a = append(a, "-Qunused-arguments")
+		// disable word wrapping in error messages
+		a = append(a, "-fmessage-length=0")
 	}
 
 	// On OS X, some of the compilers behave as if -fno-common