The new SSA backend for s390x can use R0 as a general purpose register.
This change modifies assembly code to either avoid using R0 entirely
or explicitly set R0 to 0.

R0 can still be safely used as 0 in address calculations.

Change-Id: I3efa723e9ef322a91a408bd8c31768d7858526c8
Reviewed-on: https://go-review.googlesource.com/28976Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>