Everywhere: Fix some alignment issues

When creating uninitialized storage for variables, we need to make sure
that the alignment is correct. Fixes a KUBSAN failure when running
kernels compiled with Clang.

In `Syscalls/socket.cpp`, we can simply use local variables, as
`sockaddr_un` is a POD type.

Along with moving the `alignas` specifier to the correct member,
`AK::Optional`'s internal buffer has been made non-zeroed by default.
GCC emitted bogus uninitialized memory access warnings, so we now use
`__builtin_launder` to tell the compiler that we know what we are doing.
This might disable some optimizations, but judging by how GCC failed to
notice that the memory's initialization is dependent on `m_has_value`,
I'm not sure that's a bad thing.

Kernel/Heap/kmalloc.cpp

@@ -185,7 +185,7 @@ struct KmallocGlobalHeap {
 };
 
 READONLY_AFTER_INIT static KmallocGlobalHeap* g_kmalloc_global;
-static u8 g_kmalloc_global_heap[sizeof(KmallocGlobalHeap)];
+alignas(KmallocGlobalHeap) static u8 g_kmalloc_global_heap[sizeof(KmallocGlobalHeap)];
 
 // Treat the heap as logically separate from .bss
 __attribute__((section(".heap"))) static u8 kmalloc_eternal_heap[ETERNAL_RANGE_SIZE];