Everywhere: Replace uses of __serenity__ with AK_OS_SERENITY

Now that we have OS macros for essentially every supported OS, let's try
to use them everywhere.

Userland/Libraries/LibJS/Heap/BlockAllocator.cpp

@@ -21,7 +21,7 @@ BlockAllocator::~BlockAllocator()
 {
     for (auto* block : m_blocks) {
         ASAN_UNPOISON_MEMORY_REGION(block, HeapBlock::block_size);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (munmap(block, HeapBlock::block_size) < 0) {
             perror("munmap");
             VERIFY_NOT_REACHED();
@@ -39,7 +39,7 @@ void* BlockAllocator::allocate_block([[maybe_unused]] char const* name)
         size_t random_index = get_random_uniform(m_blocks.size());
         auto* block = m_blocks.unstable_take(random_index);
         ASAN_UNPOISON_MEMORY_REGION(block, HeapBlock::block_size);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (set_mmap_name(block, HeapBlock::block_size, name) < 0) {
             perror("set_mmap_name");
             VERIFY_NOT_REACHED();
@@ -48,7 +48,7 @@ void* BlockAllocator::allocate_block([[maybe_unused]] char const* name)
         return block;
     }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto* block = (HeapBlock*)serenity_mmap(nullptr, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_RANDOMIZED | MAP_PRIVATE, 0, 0, HeapBlock::block_size, name);
     VERIFY(block != MAP_FAILED);
 #else
@@ -62,7 +62,7 @@ void BlockAllocator::deallocate_block(void* block)
 {
     VERIFY(block);
     if (m_blocks.size() >= max_cached_blocks) {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (munmap(block, HeapBlock::block_size) < 0) {
             perror("munmap");
             VERIFY_NOT_REACHED();

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -20,13 +20,13 @@
 #include <LibJS/SafeFunction.h>
 #include <setjmp.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <serenity.h>
 #endif
 
 namespace JS {
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 static int gc_perf_string_id;
 #endif
 
@@ -36,7 +36,7 @@ static __thread HashMap<FlatPtr*, size_t>* s_custom_ranges_for_conservative_scan
 Heap::Heap(VM& vm)
     : m_vm(vm)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto gc_signpost_string = "Garbage collection"sv;
     gc_perf_string_id = perf_register_string(gc_signpost_string.characters_without_null_termination(), gc_signpost_string.length());
 #endif
@@ -91,7 +91,7 @@ void Heap::collect_garbage(CollectionType collection_type, bool print_report)
     VERIFY(!m_collecting_garbage);
     TemporaryChange change(m_collecting_garbage, true);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     static size_t global_gc_counter = 0;
     perf_event(PERF_EVENT_SIGNPOST, gc_perf_string_id, global_gc_counter++);
 #endif

Userland/Libraries/LibJS/Heap/HeapBlock.cpp

@@ -20,7 +20,7 @@ namespace JS {
 
 NonnullOwnPtr<HeapBlock> HeapBlock::create_with_cell_size(Heap& heap, size_t cell_size)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     char name[64];
     snprintf(name, sizeof(name), "LibJS: HeapBlock(%zu)", cell_size);
 #else