cmd/compile: add floating point min/max intrinsics on s390x

cmd/compile: add floating point min/max intrinsics on s390x

Add the VECTOR FP (MINIMUM|MAXIMUM) instructions to the assembler and
use them in the compiler to implement min and max.

Note: I've allowed floating point registers to be used with the single
element instructions (those with the W instead of V prefix) to allow
easier integration into the compiler.

Change-Id: I5f80a510bd248cf483cce95f1979bf63fbae7de6
Reviewed-on: https://go-review.googlesource.com/c/go/+/684715
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <mark@golang.org>
Reviewed-by: Keith Randall <khr@google.com>