mac: Produce coredumps on segfault (#46157)

This changes the mach exception server to ignore fatal SIGSEGVs,
letting regular kernel processing handle it (by performing POSIX
signal delivery and then subsequently coredumping), rather than
quitting the process directly. There's probably some way to
induce the kernel to perform core dumping directly from the
exception server, but I think it'll be less confusing all around
to just have segfaults take the standard path.

Hoping this will help debug #46152.

src/signals-mach.c

@@ -167,6 +167,8 @@ typedef arm_exception_state64_t host_exception_state_t;
 #define HOST_EXCEPTION_STATE_COUNT ARM_EXCEPTION_STATE64_COUNT
 #endif
 
+#define MIG_DESTROY_REQUEST -309
+
 static void jl_call_in_state(jl_ptls_t ptls2, host_thread_state_t *state,
                              void (*fptr)(void))
 {
@@ -332,9 +334,7 @@ kern_return_t catch_mach_exception_raise(
         return KERN_SUCCESS;
     }
     else {
-        thread0_exit_count++;
-        jl_exit_thread0(128 + SIGSEGV, NULL, 0);
-        return KERN_SUCCESS;
+        return MIG_DESTROY_REQUEST;
     }
 }