port-arm/52666: ARM spinlocks are suboptimal

[coypu%sdf.org@localhost]

>Number:         52666
>Category:       port-arm
>Synopsis:       ARM spinlocks are suboptimal
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-arm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Oct 28 18:40:00 +0000 2017
>Originator:     coypu
>Release:        NetBSD 8.99.5
>Organization:
>Environment:
NetBSD 8.99.5
>Description:
newer ARM has an instruction 'wfe' essentially made for spinlocks. we should use it, but I don't see a relevant use of it, only an unused macro.

(In fact only x86 has a spinlock backoff function).
>How-To-Repeat:

>Fix: