Fixed async exceptions on arm64

The following tests - tests/lib-systhreads/eintr.ml and
tests/async-exns/async_exns_1 were failing.

This patch loads the trap ptr into Caml_state->async_exception_pointer in
arm64.S which fixes the test cases.

Author: anmolsahoo25

ocaml/runtime/arm64.S

@@ -338,15 +338,19 @@ L(jump_to_caml):
         ldr     x8, Caml_state(bottom_of_stack)
         ldr     x9, Caml_state(last_return_address)
         ldr     x10, Caml_state(gc_regs)
+        ldr     x11, Caml_state(async_exception_pointer)
         stp     x8, x9, [sp, -32]!     /* 16-byte alignment */
         CFI_ADJUST(32)
         str     x10, [sp, 16]
+        str     x11, [sp, 24]
     /* Setup a trap frame to catch exceptions escaping the OCaml code */
         ldr     x8, Caml_state(exception_pointer)
         adr     x9, L(trap_handler)
         stp     x8, x9, [sp, -16]!
         CFI_ADJUST(16)
         add     TRAP_PTR, sp, #0
+    /* Store the async exception pointer */
+        str     TRAP_PTR, Caml_state(async_exception_pointer)
     /* Reload allocation pointer */
         ldr     ALLOC_PTR, Caml_state(young_ptr)
     /* Call the OCaml code */
@@ -358,12 +362,14 @@ L(caml_retaddr):
         str     x8, Caml_state(exception_pointer)
     /* Pop the callback link, restoring the global variables */
 L(return_result):
+        ldr     x11, [sp, 24]
         ldr     x10, [sp, 16]
         ldp     x8, x9, [sp], 32
         CFI_ADJUST(-32)
         str     x8, Caml_state(bottom_of_stack)
         str     x9, Caml_state(last_return_address)
         str     x10, Caml_state(gc_regs)
+        str     x11, Caml_state(async_exception_pointer)
     /* Update allocation pointer */
         str     ALLOC_PTR, Caml_state(young_ptr)
     /* Reload callee-save registers and return address */