Kernel+LibC+LibELF: Set stack size based on PT_GNU_STACK during execve

Some programs explicitly ask for a different initial stack size than
what the OS provides. This is implemented in ELF by having a
PT_GNU_STACK header which has its p_memsz set to the amount that the
program requires. This commit implements this policy by reading the
p_memsz of the header and setting the main thread stack size to that.
ELF::Image::validate_program_headers ensures that the size attribute is
a reasonable value.

Kernel/API/serenity_limits.h

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2022, sin-ack <sin-ack@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#define PTHREAD_STACK_MAX (8 * 1024 * 1024) // 8MiB