LibC: Implement _aligned_malloc and _aligned_free

C++17 introduced aligned versions of `new` and `delete`, which are
automatically called by the compiler when allocating over-aligned
objects. As with the regular allocator functions, these are generally
thin wrappers around LibC.

We did not have support for aligned allocations in LibC, so this was not
possible. While libstdc++ has a fallback implementation, libc++ does
not, so the aligned allocation function was disabled internally. This
made building the LLVM port with Clang impossible.

Note that while the Microsoft docs say that aligned_malloc and
_aligned_free are declared in `malloc.h`, libc++ doesn't #include that
file, but instead relies on the definition coming from `stdlib.h`.
Therefore, I chose to declare it in that file instead of creating a new
LibC header.

I chose not to implement the more Unix-y `memalign`, `posix_memalign`,
or the C11 `aligned_alloc`, because that would require us to
significantly alter the memory allocator's internals. See the comment in
malloc.cpp.

Userland/Libraries/LibC/stdlib.h

@@ -25,6 +25,8 @@ size_t malloc_good_size(size_t);
 void serenity_dump_malloc_stats(void);
 void free(void*);
 __attribute__((alloc_size(2))) void* realloc(void* ptr, size_t);
+__attribute__((malloc, alloc_size(1), alloc_align(2))) void* _aligned_malloc(size_t size, size_t alignment);
+void _aligned_free(void* memblock);
 char* getenv(const char* name);
 char* secure_getenv(const char* name);
 int putenv(char*);