Gdb can be sensitive to contents of .gdbinit, and to run
this test properly needs to have runtime/runtime-gdb.py
on the auto load safe path.  Therefore, turn off .gdbinit
loading and explicitly add $GOROOT/runtime to the safe
load path.

This should make ssa/debug_test.go run more consistently.

Updates #24464.

Change-Id: I63ed17c032cb3773048713ce51fca3a3f86e79b6
Reviewed-on: https://go-review.googlesource.com/102598
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>