port-sh3/52260: SuperH needs a way to sync instruction cache

[uwe%NetBSD.org@localhost]

>Number:         52260
>Category:       port-sh3
>Synopsis:       SuperH needs a way to sync instruction cache
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-sh3-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon May 29 01:00:00 +0000 2017
>Originator:     Valery Ushakov
>Release:        NetBSD-current
>Organization:
NetBSD
>Environment:
NetBSD felix 7.99.75 NetBSD 7.99.75 (FELIX) #1: Mon May 29 02:36:09 MSK 2017  uwe@sampo:/home/uwe/work/netbsd/cvs/src/sys/arch/landisk/compile/FELIX landisk

>Description:
As far as I can tell there's no way for userland code to sync
instruction cache, which is necessary for self-modifying code.
Cf. ARM's ARM_SYNC_ICACHE sysarch(2) syscall and its arm_sync_icache()
wrapper.

NetBSD/sh3 currently doesn't provide any architecture-dependent system
calls.

>How-To-Repeat:
Stumbled into this when testing a forth I wrote with the Forth 2012 test suite that prods the system just right to make icache problem apparent.
>Fix: