cmd/compile: zeroAuto clobbers flags on s390x

[mundaym]

As discussed in CL 41076: zeroAuto is called after register allocation etc. and the flags (condition code) may be live. The current implementation of zeroAuto on s390x clobbers flags so it needs to be modified.

My plan is to modify the assembler to not clobber flags when assembling MOVD $0, <auto> (currently it might use the AGFI instruction) and change zeroAuto to use that.

[gopherbot]

CL https://golang.org/cl/42179 mentions this issue.