Really fix Windows gdbserver segment registers

Really fix Windows gdbserver segment registers

My earlier attempt to mask the segment registers in gdbserver for
Windows introduced some bugs.  That patch is here:

https://sourceware.org/pipermail/gdb-patches/2023-December/205306.html

The problem turned out to be that these fields in the Windows
'CONTEXT' type are just 16 bits, so while masking the values on read
is fine, writing the truncated values back then causes zeros to be
written to some subsequent field.

This patch cleans this up by arranging never to write too much data to
a field.

I also noticed that two register numbers here were never updated for
the 64-bit port.  This patch fixes this as well, and renames the "FCS"
register to follow gdb.

Finally, this patch applies the same treatment to windows-nat.c.

Reviewed-by: John Baldwin <jhb@FreeBSD.org>