Autogenerate gdbarch doc for internals manual

[Stan Shebs <stan@codesourcery.com>]

Undeterred by the stunning lack of response to my last internals manuals 
query (http://sourceware.org/ml/gdb/2008-07/msg00309.html, not too late 
to speak up :-) ), I bring up an idea suggested on irc, which is to 
generate the internals manual's detailed description of gdbarch methods 
from gdbarch.sh . Although I'm not generally a fan of autogenerated docs 
- I find they tend to be heavy on syntax, and light on semantics - the 
internals manual has fallen way behind what is actually in gdbarch, and 
there are other manual sections that can talk more about how all the 
different methods work together.

Mechanically, the way I see it working is that running gdbarch.sh 
produces a third file, doc/gdbarch.texi, which is then included in 
doc/gdbint.texinfo. Some gdbint.texinfo bits will migrate into 
gdbarch.sh; I don't think there will be a problem including texinfo 
markup in gdbarch.sh, just need basic @foo{} constructs to get passed 
through. This is going to be more of a background task for me, but I 
wanted to get some agreement on the direction before starting to tinker.

Stan