R14, R15 must be saved in sigtramp because they might be modified by Go
code when a SIGPROF occurs.

Fixes #28555

Change-Id: I573541f108d7d6aac8e60d33c649e5db943f3ef5
Reviewed-on: https://go-review.googlesource.com/c/go/+/169117
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>