gdbserver/x86: move no-xml code earlier in x86_linux_read_description

This commit is part of a series that aims to share more of the x86
target description reading/generation code between GDB and gdbserver.

There are a huge number of similarities between the code in
gdbserver's x86_linux_read_description function and GDB's
x86_linux_nat_target::read_description function, and it is this
similarity that I plan, in a later commit, to share between GDB and
gdbserver.

However, one thing that is different in x86_linux_read_description is
the code inside the '!use_xml' block.  This is the code that handles
the case where gdbserver is not allowed to send an XML target
description back to GDB.  In this case gdbserver uses some predefined,
fixed, target descriptions.

First, it's worth noting that I suspect this code is not tested any
more.  I couldn't find anything in the testsuite that tries to disable
XML target description support.  And the idea of having a single
"fixed" target description really doesn't work well when we think
about all the various x86 extensions that exist.  Part of me would
like to rip out the no-xml support in gdbserver (at least for x86),
and if a GDB connects that doesn't support XML target descriptions,
gdbserver can just give an error and drop the connection.  GDB has
supported XML target descriptions for 16 years now, I think it would
be reasonable for our shipped gdbserver to drop support for the old
way of doing things.

Anyway.... this commit doesn't do that.

What I did notice was that, over time, the '!use_xml' block appears to
have "drifted" within the x86_linux_read_description function; it's
now not the first check we do.  Instead we make some ptrace calls and
return a target description generated based on the result of these
ptrace calls.  Surely it only makes sense to generate variable target
descriptions if we can send these back to GDB?

So in this commit I propose to move the '!use_xml' block earlier in
the x86_linux_read_description function.

The benefit of this is that this leaves the later half of
x86_linux_read_description much more similar to the GDB function
x86_linux_nat_target::read_description and sets us up for potentially
sharing code between GDB and gdbserver in a later commit.

Approved-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>

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