runtime: fix ppc64 startup on newer linux kernels

runtime: fix ppc64 startup on newer linux kernels

R0 needs to be cleared at startup as it may not always be cleared
by the kernel on newer kernels.

Fixes #54547

Change-Id: Id7055699aaa8d8b193b7e3e784f075ce29ac3f1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/424927
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>