When instruction has only one argument, Go parser saves the
argument value into prog.From without any special handling.
But assembler gets the argument value from prog.To.

The fix adds special handling for CLREX and puts other instructions
arguments value into prog.From.

Uncomment hlt/hvc/smc/brk/dcps1/dcps2/dcps3/clrex test cases.

Fixes #20765

Change-Id: I1fc0d2faafb19b537cab5a665bd4af56c3a2c925
Reviewed-on: https://go-review.googlesource.com/78275
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>