Make auxv parsing in linux/arm less of a special case.

* rename setup_auxv to sysargs
* exclude linux/arm from vdso_none.go
* move runtime.checkarm after runtime.sysargs so arm specific
  values are properly initialised

Change-Id: I1ca7f5844ad5a162337ff061a83933fc9a2b5ff6
Reviewed-on: https://go-review.googlesource.com/2681Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>