chips/samd21: Prepare for picolibc stdio change

Picolibc is changing how stdio is initialized, removing the __iob
array and using three globals (stdin/stdout/stderr). Prepare for that
by allowing either new or old libraries to work with snek.

Signed-off-by: Keith Packard <keithp@keithp.com>

chips/samd21/ao-stdio.c

@@ -22,4 +22,17 @@
 
 static FILE __stdio = FDEV_SETUP_STREAM(ao_usb_putc, snek_io_getc, ao_usb_flush, _FDEV_SETUP_RW);
 
+#ifdef PICOLIBC_STDIO_GLOBALS
+#ifdef __strong_reference
+#define STDIO_ALIAS(x) __strong_reference(stdin, x);
+#else
+#define STDIO_ALIAS(x) FILE *const x = &__stdio;
+#endif
+
+FILE *const stdin = &__stdio;
+STDIO_ALIAS(stdout);
+STDIO_ALIAS(stderr);
+
+#else
 FILE *const __iob[3] = { &__stdio, &__stdio, &__stdio };
+#endif

chips/samd21/snek-eeprom.c

@@ -73,8 +73,8 @@ snek_eeprom_load(void)
 {
 	snek_interactive = false;
 	ao_flash_read_init();
-	save_getc = __iob[0]->get;
-	__iob[0]->get = snek_eeprom_getchar;
+	save_getc = stdin->get;
+	stdin->get = snek_eeprom_getchar;
 }
 
 int
@@ -87,6 +87,6 @@ snek_eeprom_getchar(FILE *stream)
 			return c;
 	}
 	snek_interactive = true;
-	__iob[0]->get = save_getc;
+	stdin->get = save_getc;
 	return EOF;
 }