Use the right register type in arm64

As we were using thhe x86 one, `bpf_user_pt_regs_t` is defined in
vmlinux as:

```
typedef struct pt_regs bpf_user_pt_regs_t;
```

in x86 and:

```
typedef struct user_pt_regs bpf_user_pt_regs_t;
```

in arm64

Test Plan
=========

Tested on my x86 and arm64 machines.

src/bpf/profiler.bpf.c

@@ -560,7 +560,7 @@ int dwarf_unwind(struct bpf_perf_event_data *ctx) {
 }
 
 // Set up the initial unwinding state.
-static __always_inline bool set_initial_state(unwind_state_t *unwind_state, struct pt_regs *regs) {
+static __always_inline bool set_initial_state(unwind_state_t *unwind_state, bpf_user_pt_regs_t *regs) {
   unwind_state->stack.len = 0;
   unwind_state->tail_calls = 0;