cmd: remove a few more unused parameters

cmd: remove a few more unused parameters

ssa's pos parameter on the Const* funcs is unused, so remove it.

ld's alloc parameter on elfnote is always true, so remove the arguments
and simplify the code.

Finally, arm's addpltreloc never has its return parameter used, so
remove it.

Change-Id: I63387ecf6ab7b5f7c20df36be823322bb98427b8
Reviewed-on: https://go-review.googlesource.com/104456
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>