compiler,runtime: use __builtin_dwarf_cfa for getcallersp

compiler,runtime: use __builtin_dwarf_cfa for getcallersp

    Currently, the compiler lowers runtime.getcallersp to
    __builtin_frame_address(1). In the C side of the runtime,
    getcallersp is defined as __builtin_frame_address(0). They don't
    match. Further, neither of them actually returns the caller's SP.
    On AMD64, __builtin_frame_address(0) just returns the frame
    pointer. __builtin_frame_address(1) returns the memory content
    where the frame pointer points to, which is typically the
    caller's frame pointer but can also be garbage if the frame
    pointer is not enabled.

    This CL changes it to use __builtin_dwarf_cfa(), which returns
    the caller's SP at the call site. This matches the SP we get
    from unwinding the stack.

    Currently getcallersp is not used for anything real. It will be
    used for precise stack scan (a new version of CL 159098).

    Reviewed-on: https://go-review.googlesource.com/c/162905

	* go-gcc.cc (Gcc_backend::Gcc_backend): Define __builtin_dwarf_cfa
	instead of __builtin_frame_address.

From-SVN: r268952