sys/picolibc_syscalls_default: fix linking with 1.8.10+

This adapts to an API change for providing stdin/stdout/stderr: The
macro to test whether globals are to be used is now prefixed with two
underscores.

Co-authored-by: crasbe <[email protected]>

Author: maribu

sys/picolibc_syscalls_default/syscalls.c

@@ -238,7 +238,9 @@ static int picolibc_get(FILE *file)
 FILE picolibc_stdio =
     FDEV_SETUP_STREAM(picolibc_put, picolibc_get, picolibc_flush, _FDEV_SETUP_RW);
 
-#ifdef PICOLIBC_STDIO_GLOBALS
+/* Since picolibc 1.8.10, PICOLIBC_STDIO_GLOBALS is prefixed with two leading
+ * underscores. We just test for both to remain backwards compatible */
+#if defined(PICOLIBC_STDIO_GLOBALS) || defined(__PICOLIBC_STDIO_GLOBALS)
 #ifdef __strong_reference
 /* This saves two const pointers.
  * See https://github.com/RIOT-OS/RIOT/pull/17001#issuecomment-945936918