cmd/asm: add masked branch and conditional load instructions to s390x

The branch-relative-on-condition (BRC) instruction allows us to use
an immediate to specify under what conditions the branch is taken.
For example, `BRC $7, L1` is equivalent to `BNE L1`. It is sometimes
useful to specify branches in this way when either we don't have
an extended mnemonic for a particular mask value or we want to
generate the condition code mask programmatically.

The new load-on-condition (LOCR and LOCGR) and compare-and-branch
(CRJ, CGRJ, CLRJ, CLGRJ, CIJ, CGIJ, CLIJ and CLGIJ) instructions
provide the same flexibility for conditional loads and combined
compare and branch instructions.

Change-Id: Ic6f5d399b0157e278b39bd3645f4ee0f4df8e5fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/196558


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>