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.

Userland/Libraries/LibC/stdio.cpp

@@ -614,7 +614,7 @@ private:
 
 extern "C" {
 
-static u8 default_streams[3][sizeof(FILE)];
+alignas(FILE) static u8 default_streams[3][sizeof(FILE)];
 FILE* stdin = reinterpret_cast<FILE*>(&default_streams[0]);
 FILE* stdout = reinterpret_cast<FILE*>(&default_streams[1]);
 FILE* stderr = reinterpret_cast<FILE*>(&default_streams[2]);