include/exec: Move gdb open flags to gdbstub.h

There were 3 copies of these flags.  Place them in the
file with gdb_do_syscall, with which they belong.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
4 files changed