loader: additional fixes for Arduino_JSON

Minor fixes for Arduino_JSON library compatibility:

- '__assert_no_args' and 'exit' are actually libc symbols, so the guards
  prevented their export in certain situations.

- _exit, stdin, stdout, stderr symbols are also exported to cover the
  dynamic link case.

- an 'undefined reference to _ctype_' error occurred because the libc is
  compiled with -Os, while the sketches are NOT - this creates an issue
  because the size optimization auto-selects a different version of
  ctype functions in picolibc. -D_PICOLIBC_CTYPE_SMALL=1 forces the
  small version to be used.

Note by committer: Only under the `loader` was picked.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>
Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

Author: pillo79

loader/llext_exports.c

@@ -195,7 +195,6 @@ FORCE_EXPORT_SYM(close);
 FORCE_EXPORT_SYM(accept);
 FORCE_EXPORT_SYM(bind);
 FORCE_EXPORT_SYM(listen);
-EXPORT_SYMBOL(exit);
 FORCE_EXPORT_SYM(inet_pton);
 FORCE_EXPORT_SYM(sendto);
 FORCE_EXPORT_SYM(recvfrom);
@@ -241,6 +240,12 @@ EXPORT_SYMBOL(cbvprintf);
 EXPORT_SYMBOL(vsnprintf);
 FORCE_EXPORT_SYM(abort);
 EXPORT_SYMBOL(sscanf);
+EXPORT_SYMBOL(exit);
+FORCE_EXPORT_SYM(_exit);
+FORCE_EXPORT_SYM(__assert_no_args);
+EXPORT_SYMBOL(stdin);
+EXPORT_SYMBOL(stdout);
+EXPORT_SYMBOL(stderr);
 
 #if defined(CONFIG_RING_BUFFER)
 EXPORT_SYMBOL(ring_buf_get);
@@ -283,7 +288,6 @@ FORCE_EXPORT_SYM(__aeabi_dcmpge);
 
 #if defined (CONFIG_CPP)
 FORCE_EXPORT_SYM(__cxa_pure_virtual);
-FORCE_EXPORT_SYM(__assert_no_args);
 #endif
 
 #if defined(CONFIG_BOARD_ARDUINO_UNO_Q)