shared/runtime/semihosting_arm: Add mp_semihosting_exit.

Signed-off-by: Damien George <damien@micropython.org>

shared/runtime/semihosting_arm.c

@@ -36,6 +36,7 @@
 #define SYS_WRITE  0x05
 #define SYS_READ   0x06
 #define SYS_READC  0x07
+#define SYS_EXIT   0x18
 
 // Constants:
 #define OPEN_MODE_READ  (0) // mode "r"
@@ -88,6 +89,13 @@ void mp_semihosting_init() {
     mp_semihosting_stdout = mp_semihosting_open_console(OPEN_MODE_WRITE);
 }
 
+void mp_semihosting_exit(int status) {
+    if (status == 0) {
+        status = 0x20026;
+    }
+    mp_semihosting_call(SYS_EXIT, (void *)(uintptr_t)status);
+}
+
 int mp_semihosting_rx_char() {
     return mp_semihosting_call(SYS_READC, NULL);
 }

shared/runtime/semihosting_arm.h

@@ -46,6 +46,7 @@ If mp_semihosting_rx_char() doesn't work then try mp_semihosting_rx_chars(str, 1
 #include <stdint.h>
 
 void mp_semihosting_init();
+void mp_semihosting_exit(int status);
 int mp_semihosting_rx_char();
 int mp_semihosting_rx_chars(char *str, size_t len);
 uint32_t mp_semihosting_tx_strn(const char *str, size_t len);