Meta: Disable -Wmaybe-uninitialized

It's prone to finding "technically uninitialized but can never happen"
cases, particularly in Optional<T> and Variant<Ts...>.
The general case seems to be that it cannot infer the dependency
between Variant's index (or Optional's boolean state) and a particular
alternative (or Optional's buffer) being untouched.
So it can flag cases like this:
```c++
if (index == StaticIndexForF)
    new (new_buffer) F(move(*bit_cast<F*>(old_buffer)));
```
The code in that branch can _technically_ make a partially initialized
`F`, but that path can never be taken since the buffer holding an
object of type `F` and the condition being true are correlated, and so
will never be taken _unless_ the buffer holds an object of type `F`.

This commit also removed the various 'diagnostic ignored' pragmas used
to work around this warning, as they no longer do anything.

Kernel/Syscall.cpp

@@ -120,15 +120,7 @@ KResultOr<FlatPtr> handle(RegisterState& regs, FlatPtr function, FlatPtr arg1, F
         return ENOSYS;
     }
 
-    // This appears to be a bogus warning, as s_syscall_table is always
-    // initialized, and the index (function) is always bounded.
-    // TODO: Figure out how to avoid the suppression.
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
-
     return (process.*(s_syscall_table[function]))(arg1, arg2, arg3);
-
-#pragma GCC diagnostic pop
 }
 
 }