runtime: fix context used by getTraceback

    
    I typoed the argument passed to getcontext in getTraceback, and the
    error was hidden by ucontext_arg.  This would have been caught by some
    of the runtime package tests, but we don't run most of them because
    they rely on `go build`, and the go tool is not available while
    running the libgo testsuite.  We should fix the libgo testsuite,
    somehow, so that they run.
    
    Reviewed-on: https://go-review.googlesource.com/35837

From-SVN: r244946

gcc/go/gofrontend/MERGE

-5c6c93f58e2aaae186bac5dcde9df1679d4896b1
+f8c451ed42fd71b85afab54a33fc321df0ff3b0b
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/proc.c

@@ -631,7 +631,7 @@ void getTraceback(G* me, G* gp)
 #ifdef USING_SPLIT_STACK
 	__splitstack_getcontext(&me->stackcontext[0]);
 #endif
-	getcontext(ucontext_arg(&me->stackcontext[0]));
+	getcontext(ucontext_arg(&me->context[0]));
 
 	if (gp->traceback != nil) {
 		runtime_gogo(gp);