AK+Kernel: Make fallible allocations compiler-agnostic

In standard C++, operators `new` and `new[]` are guaranteed to return a
valid (non-null) pointer and throw an exception if the allocation
couldn't be performed. Based on this, compilers did not check the
returned pointer before attempting to use them for object construction.

To avoid this, the allocator operators were changed to be `noexcept` in
PR #7026, which made GCC emit the desired null checks. Unfortunately,
this is a non-standard feature which meant that Clang would not accept
these function definitions, as it did not match its expected
declaration.

To make compiling using Clang possible, the special "nothrow" versions
of `new` are implemented in this commit. These take a tag type of
`std::nothrow_t` (used for disambiguating from placement new/etc.), and
are allowed by the standard to return null. There is a global variable,
`std::nothrow`, declared with this type, which is also exported into the
global namespace.

To perform fallible allocations, the following syntax should be used:

```cpp
auto ptr = new (nothrow) T;
```

As we don't support exceptions in the kernel, the only way of uphold the
"throwing" new's guarantee is to abort if the allocation couldn't be
performed. Once we have proper OOM handling in the kernel, this should
only be used for critical allocations, where we wouldn't be able to
recover from allocation failures anyway.

Kernel/Heap/kmalloc.cpp

@@ -29,6 +29,10 @@
 #define POOL_SIZE (2 * MiB)
 #define ETERNAL_RANGE_SIZE (2 * MiB)
 
+namespace std {
+const nothrow_t nothrow;
+}
+
 static RecursiveSpinLock s_lock; // needs to be recursive because of dump_backtrace()
 
 static void kmalloc_allocate_backup_memory();
@@ -300,12 +304,26 @@ size_t kmalloc_good_size(size_t size)
     return size;
 }
 
-void* operator new(size_t size) noexcept
+void* operator new(size_t size)
+{
+    void* ptr = kmalloc(size);
+    VERIFY(ptr);
+    return ptr;
+}
+
+void* operator new(size_t size, const std::nothrow_t&) noexcept
 {
     return kmalloc(size);
 }
 
-void* operator new[](size_t size) noexcept
+void* operator new[](size_t size)
+{
+    void* ptr = kmalloc(size);
+    VERIFY(ptr);
+    return ptr;
+}
+
+void* operator new[](size_t size, const std::nothrow_t&) noexcept
 {
     return kmalloc(size);
 }