Fix constness problem in ioscm_make_gdb_stdio_port

Fix constness problem in ioscm_make_gdb_stdio_port

ioscm_make_gdb_stdio_port passes const char pointers (literal strings) to
scm_mode_bits, which takes a non-const char pointer.  Ideally, we would
change scm_mode_bits to take a const char pointer, but it's not part of
an API we control.

Instead, it's easy enough to build the string to pass to scm_mode_bits in
a (non-const) char array and pass that.

gdb/ChangeLog:

	* guile/scm-ports.c (ioscm_make_gdb_stdio_port): Pass non-const
	char pointer to scm_mode_bits.