Treat attributes as code in DWARF assembler

Treat attributes as code in DWARF assembler

The DWARF assembler treats the 'children' of a DIE as plain Tcl code,
evaluating it in the parent context.

I don't recall why, but when I wrote this code, I didn't do the same
thing for the attributes.  Instead, there I implemented a special
syntax.  I was looking at this today and wondered why I didn't just
use ordinary evaluation as well.

This patch implements this idea.  Attributes are now evaluated as
plain code.  This is a bit less "magical", is slightly shorter due to
lack of braces, and most importantly now allows comments in the
attributes section.

Note that some [subst {}] calls had to be added.  This could be fixed
by changing DWARF expressions to also be plain Tcl code.  I think that
would be a good idea, but I didn't want to tack it on here.

This patch requires the full ("DW_AT_...") name for attributes.  I did
this to avoid any possibility of name clashes.  I've long considered
that my original decision to allow short names for tags and attributes
was a mistake.  It's worth noting that many existing tests already
used the long names here.

Most of this patch was written by script.  The main changes are in
dwarf.exp, but as noted, there were some minor fixups needed in some
tests.

Also, after committing, 'git show' indicated some whitespace issues,
so I've gone through and "tabified" some things, which is why the
patch might be otherwise larger than it should be.  (This was
discussed a bit during the v1 submission.)

v1 was here:

https://inbox.sourceware.org/gdb-patches/20250530183845.2179955-1-tromey@adacore.com/

In v2 I've rebased and fixed up various tests that either changed or
were added since v1.

Regression tested on x86-64 Fedora 41.

Approved-By: Simon Marchi <simon.marchi@efficios.com>