gdb/x86: move reading of cs and ds state into gdb/nat directory

This patch is part of a series that has the aim sharing the x86 Linux
target description creation code between GDB and gdbserver.

Within GDB part of this process involves reading the cs and ds state
from the 'struct user_regs_struct' using a ptrace call.

This isn't done by gdbserver, which is part of the motivation for this
whole series; the approach gdbserver takes is inferior to the approach
GDB takes (gdbserver relies on reading the file being debugged, and
extracting similar information from the file headers).

This commit moves the reading of cs and ds, which is used to figure
out if a thread is 32-bit or 64-bit (or in x32 mode), into the gdb/nat
directory so that the code can be shared with gdbserver, but at this
point I'm not actually using the code in gdbserver, that will come
later.

As such there should be no user visible changes after this commit, GDB
continues to do things as it did before (reading cs/ds), while
gdbserver continues to use its own approach (which doesn't require
reading cs/ds).

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