runtime: fix context used by getTraceback

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