Gdb is not able to backtrace our non-standard stack frames on RISC
architectures without frame pointer.

Change-Id: Id62a566ce2d743602ded2da22ff77b9ae34bc5ae
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/9456Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>