Kernel: Use Process::require_promise() instead of REQUIRE_PROMISE()

This change lays the foundation for making the require_promise return
an error hand handling the process abort outside of the syscall
implementations, to avoid cases where we would leak resources.

It also has the advantage that it makes removes a gs pointer read
to look up the current thread, then process for every syscall. We
can instead go through the Process this pointer in most cases.

Kernel/Syscalls/getrandom.cpp

@@ -16,7 +16,7 @@ namespace Kernel {
 ErrorOr<FlatPtr> Process::sys$getrandom(Userspace<void*> buffer, size_t buffer_size, [[maybe_unused]] unsigned flags)
 {
     VERIFY_NO_PROCESS_BIG_LOCK(this);
-    REQUIRE_PROMISE(stdio);
+    require_promise(Pledge::stdio);
     if (buffer_size > NumericLimits<ssize_t>::max())
         return EINVAL;