py/nlraarch64: Fix dangerous use of input register.

Starting with 2757acf, the `top` variable in `nlr_jump()` in
`nlraarch64.c` was assigned to register `x19` by the compiler.  However,
the assembly code writes over that register with

    ldp x19, x20, [%0,  micropython#32]

since `%0` is now `x19`. This causes the next line

    ldp lr,  x9,  [%0,  micropython#16]

to load the wrong values.

To fix the issue, we move the value of the `top` variable from an unknown
register to a known register at the beginning of the asm code then only use
known/hard-coded registers after that.

Fixes issue micropython#11754.

Signed-off-by: David Lechner <[email protected]>

py/nlraarch64.c

@@ -62,13 +62,14 @@ NORETURN void nlr_jump(void *val) {
     MP_STATIC_ASSERT(offsetof(nlr_buf_t, regs) == 16); // asm assumes it
 
     __asm volatile (
-        "ldr x29,      [%0, #112]\n"
-        "ldp x27, x28, [%0,  #96]\n"
-        "ldp x25, x26, [%0,  #80]\n"
-        "ldp x23, x24, [%0,  #64]\n"
-        "ldp x21, x22, [%0,  #48]\n"
-        "ldp x19, x20, [%0,  #32]\n"
-        "ldp lr,  x9,  [%0,  #16]\n" // 16 == offsetof(nlr_buf_t, regs)
+        "mov x0, %0              \n"
+        "ldr x29,      [x0, #112]\n"
+        "ldp x27, x28, [x0,  #96]\n"
+        "ldp x25, x26, [x0,  #80]\n"
+        "ldp x23, x24, [x0,  #64]\n"
+        "ldp x21, x22, [x0,  #48]\n"
+        "ldp x19, x20, [x0,  #32]\n"
+        "ldp lr,  x9,  [x0,  #16]\n" // 16 == offsetof(nlr_buf_t, regs)
         "mov sp, x9              \n"
         "mov x0, #1              \n"  // non-local return
         "ret                     \n"