Kernel: Support thread-local storage

This patch adds support for TLS according to the x86 System V ABI.
Each thread gets a thread-specific memory region, and the GS segment
register always points _to a pointer_ to the thread-specific memory.

In other words, to access thread-local variables, userspace programs
start by dereferencing the pointer at [gs:0].

The Process keeps a master copy of the TLS segment that new threads
should use, and when a new thread is created, they get a copy of it.
It's basically whatever the PT_TLS program header in the ELF says.

Kernel/Scheduler.cpp

@@ -460,6 +460,12 @@ bool Scheduler::context_switch(Thread& thread)
         descriptor.descriptor_type = 0;
     }
 
+    if (!thread.thread_specific_data().is_null()) {
+        auto& descriptor = thread_specific_descriptor();
+        descriptor.set_base(thread.thread_specific_data().as_ptr());
+        descriptor.set_limit(sizeof(ThreadSpecificData*));
+    }
+
     auto& descriptor = get_gdt_entry(thread.selector());
     descriptor.type = 11; // Busy TSS
     flush_gdt();