Use returns_twice attribute to preserve regs in nlrthumb nlr_push()

Author: dhalbert

py/nlrthumb.c

@@ -36,7 +36,7 @@
 // For reference, arm/thumb callee save regs are:
 //      r4-r11, r13=sp
 
-__attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
+__attribute__((naked, returns_twice)) unsigned int nlr_push(nlr_buf_t *nlr) {
 
     __asm volatile (
         "str    r4, [r0, #12]       \n" // store r4 into nlr_buf