x86_64-windows GDB crash due to fs_base/gs_base registers

[Joel Brobecker <brobecker@adacore.com>]

Hello,

Here are two patches aimed at fixing a crash in GDB when debugging
on native windows targets. The crash only occurs when trying to access
the fs_base/gs_base register, but is fairly easy to trigger, as any
function call will cause that. "info reg" will also trigger it.

When I saw this failure, it made me realize that I found the current
code to be quite fragile, and I wasn't surprised once I understood
the situation that it eventually lead to this crash. As a result,
I decided to implement the register fetch/store code a bit differently.
As such, I did the work in two steps, hence the two patches.

    [PATCH 1/2] Minor reorganization of fetch_registers/store_registers
    [PATCH 2/2] x86_64-windows GDB crash due to fs_base/gs_base registers

Both patches were tested on x86-windows and x86_64-windows, using
AdaCore's testsuite.

Let me know what you think of the approach and choices taken in those
two patches.

Thanks!
-- 
Joel