target/i386: use gdb_get_reg helpers

This is cleaner than poking memory directly and will make later
clean-ups easier.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200316172155.971-14-alex.bennee@linaro.org>