Fix stack backtraces on 26-bit ARM

[Richard Earnshaw]

> Hmm, I have the bones of a completely clean NetBSD/arm port.  It isn't 
> complete, but it was written without reference to the existing NetBSD code.
> 

Ok, attached is most of the code.  There's one more patch, which I'll send
separately, that allows ARM targets to be single-stepped when there is no
kernel/hw support for single stepping.

R.

<date> Richard Earnshaw <rearnsha@arm.com>

	* config/arm/nbsd.mh config/arm/nbsd.mt config/arm/tm-nbsd.h
	config/arm/nm-nbsd.h config/arm/xm-nbsd.h: New files.
	* armbsd-nat.c: New file.
	* Makefile.in: Build it.
	* configure.host configure.tgt: Support NetBSD/arm.