For async preemption, we will be using REGTMP as a temporary
register in injected call on S390X, which will clobber it. So any
code that uses REGTMP is not safe for async preemption.

In the assembler backend, we expand a Prog to multiple machine
instructions and use REGTMP as a temporary register if necessary.
These need to be marked unsafe. Unlike ARM64 and MIPS,
instructions on S390X are variable length so we don't use the
length as a condition. Instead, we set a bit on the Prog whenever
REGTMP is used.

Change-Id: Ie5d14068a950f4c7cea51dff2c4a8bdc19ec9348
Reviewed-on: https://go-review.googlesource.com/c/go/+/204105
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>