Utilize bitshifts correctly in signals-mach.c when storing/reading the previous GC state (#53868)

I have not succeed in writing a test for this, but this was found on a
CI hang together with @Keno and @vtjnash.

In essence if we hit a safepoint while GC_SAFE things can go wrong
<img width="748" alt="image"
src="https://github.com/JuliaLang/julia/assets/28694980/7d8170ee-11ab-43de-9bb1-9219aa5a2d80">

Author: gbaraldi

src/signals-mach.c

@@ -9,6 +9,7 @@
 #include <mach/task.h>
 #include <mach/mig_errors.h>
 #include <AvailabilityMacros.h>
+#include <stdint.h>
 #include "mach_excServer.c"
 
 #ifdef MAC_OS_X_VERSION_10_9
@@ -47,14 +48,31 @@ static arraylist_t suspended_threads;
 extern uv_mutex_t safepoint_lock;
 extern uv_cond_t safepoint_cond_begin;
 
+#define GC_STATE_SHIFT 8*sizeof(int16_t)
+static inline int8_t decode_gc_state(uintptr_t item)
+{
+    return (int8_t)(item >> GC_STATE_SHIFT);
+}
+
+static inline int16_t decode_tid(uintptr_t item)
+{
+    return (int16_t)item;
+}
+
+static inline uintptr_t encode_item(int16_t tid, int8_t gc_state)
+{
+    return (uintptr_t)tid | ((uintptr_t)gc_state << GC_STATE_SHIFT);
+}
+
 // see jl_safepoint_wait_thread_resume
 void jl_safepoint_resume_thread_mach(jl_ptls_t ptls2, int16_t tid2)
 {
     // must be called with uv_mutex_lock(&safepoint_lock) and uv_mutex_lock(&ptls2->sleep_lock) held (in that order)
     for (size_t i = 0; i < suspended_threads.len; i++) {
         uintptr_t item = (uintptr_t)suspended_threads.items[i];
-        int16_t tid = (int16_t)item;
-        int8_t gc_state = (int8_t)(item >> 8);
+
+        int16_t tid = decode_tid(item);
+        int8_t gc_state = decode_gc_state(item);
         if (tid != tid2)
             continue;
         jl_atomic_store_release(&ptls2->gc_state, gc_state);
@@ -71,8 +89,8 @@ void jl_mach_gc_end(void)
     size_t j = 0;
     for (size_t i = 0; i < suspended_threads.len; i++) {
         uintptr_t item = (uintptr_t)suspended_threads.items[i];
-        int16_t tid = (int16_t)item;
-        int8_t gc_state = (int8_t)(item >> 8);
+        int16_t tid = decode_tid(item);
+        int8_t gc_state = decode_gc_state(item);
         jl_ptls_t ptls2 = jl_atomic_load_relaxed(&jl_all_tls_states)[tid];
         uv_mutex_lock(&ptls2->sleep_lock);
         if (jl_atomic_load_relaxed(&ptls2->suspend_count) == 0) {
@@ -117,7 +135,7 @@ static void jl_mach_gc_wait(jl_ptls_t ptls2, mach_port_t thread, int16_t tid)
         // triggers a SIGSEGV and gets handled by the usual codepath for unix.
         int8_t gc_state = jl_atomic_load_acquire(&ptls2->gc_state);
         jl_atomic_store_release(&ptls2->gc_state, JL_GC_STATE_WAITING);
-        uintptr_t item = tid | (((uintptr_t)gc_state) << 16);
+        uintptr_t item = encode_item(tid, gc_state);
         arraylist_push(&suspended_threads, (void*)item);
         thread_suspend(thread);
     }