syscall, runtime/internal/syscall: always return 0 in r2 on ppc64{,le} linux syscalls

Both endians perform syscalls similarly. Only CR0S0 and R3 hold
the resultant status of a syscall. A random value may be stored into
the second return value (r2) result in some cases. Always set it to
zero.

Fixes #51192

Change-Id: Ida6a5692578d2cdadf3099af28478b3bc364f623
Reviewed-on: https://go-review.googlesource.com/c/go/+/385796


Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Trust: Paul Murphy <murp@ibm.com>