tools/gdbsx: fix 'g' packet response for 64bit guests

tools/gdbsx: fix 'g' packet response for 64bit guests

gdb 8.0 fixed bounds checking for 'g' packet (commit
9dc193c3be85aafa60ceff57d3b0430af607b4ce "Check for truncated
registers in process_g_packet"). This revealed that gdbsx did
not properly formatted 'g' packet - segment registers and eflags are
expected to be 32-bit fields in the response (according to
gdb/features/i386/64bit-core.xml in gdb sources). Specific error is:

    Truncated register 26 in remote 'g' packet

instead of silently truncating part of register.

Additionally, it looks like segment registers of 64bit guests were never
reported correctly, because of type mismatch.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>