cmd/asm: add LCDBR instruction on s390x

cmd/asm: add LCDBR instruction on s390x

This CL is to add LCDBR assembly instruction mnemonics, mainly used in math package.

The LCDBR instruction has the same effect as the FNEG pseudo-instructions, just that it sets the flag.

Change-Id: I3f00f1ed19148d074c3b6c5f64af0772289f2802
Reviewed-on: https://go-review.googlesource.com/c/go/+/648036
Reviewed-by: Srinivas Pokala <Pokala.Srinivas@ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Munday <mike.munday@lowrisc.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Munday <mike.munday@lowrisc.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>