runtime: avoid write barriers with traceback info

runtime: avoid write barriers with traceback info

    Unlike the gc runtime, libgo stores traceback information in location
    structs, which contain strings.  Therefore, copying location structs
    around appears to require write barriers, although in fact write
    barriers are never important because the strings are never allocated
    in Go memory.  They come from libbacktrace.

    Some of the generated write barriers come at times when write barriers
    are not permitted.  For example, exitsyscall, marked
    nowritebarrierrec, calls exitsyscallfast which calls traceGoSysExit
    which calls traceEvent which calls traceStackID which calls
    trace.stackTab.put which copies location values into memory allocated
    by tab.newStack.  This write barrier can be invoked when there is no
    p, causing a crash.

    This change fixes the problem by ensuring that location values are
    copied around in the tracing code with no write barriers.

    This was found by fixing the compiler to fully implement
    //go:nowritebarrierrec; CL to follow.

    Reviewed-on: https://go-review.googlesource.com/134226

From-SVN: r264282