Segment register reading on Windows targets.

Segment register reading on Windows targets.

This patch makes sure that the value of segment registers are
read properly as 16bit values on Windows.

gdb/ChangeLog:

        * windows-nat.h (segment_register_p_ftype): New typedef.
        (windows_set_segment_register_p): Add declaration.
        * windows-nat.c (segment_register_p): New static global.
        (windows_set_segment_register_p): New function.
        (do_windows_fetch_inferior_registers): Add special handling
        for segment registers.
        * amd64-windows-nat.c: #include "amd64-tdep.h".
        (amd64_windows_segment_register_p): New function.
        (_initialize_amd64_windows_nat): Call windows_set_segment_register_p.
        * i386-windows-nat.c: #include "i386-tdep.h".
        (i386_windows_segment_register_p): New function.
        (_initialize_i386_windows_nat): Call windows_set_segment_register_p.