runtime: avoid register clobber in s390x racecall

runtime: avoid register clobber in s390x racecall

This is a regression in CL 643875. Loading gsignal clobbers R8, which
contains the m pointer needed for loading g0.

For #71395.

Change-Id: I6a6a636ca95442767efe0eb1b358f2139d18c5b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/675035
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>