shared/readline: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register the readline_history root
pointer array used by shared/readline.c and removes the registration from
all mpconfigport.h files.

This also required adding a new MICROPY_READLINE_HISTORY_SIZE config option
since not all ports used the same sized array.

Signed-off-by: David Lechner <david@pybricks.com>

ports/powerpc/Makefile

@@ -42,6 +42,8 @@ SRC_C = \
 	shared/runtime/stdout_helpers.c \
 	$(BUILD)/_frozen_mpy.c \
 
+SRC_QSTR += shared/readline/readline.c
+
 OBJ = $(PY_CORE_O)
 OBJ += $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 OBJ += $(BUILD)/head.o

ports/powerpc/mpconfigport.h

@@ -112,9 +112,6 @@ typedef long mp_off_t;
 
 #define MP_STATE_PORT MP_STATE_VM
 
-#define MICROPY_PORT_ROOT_POINTERS \
-    const char *readline_hist[8];
-
 // powerpc64 gcc doesn't seem to define these
 // These are pointers, so make them 64 bit types
 typedef long intptr_t;