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/Process.h

@@ -369,6 +369,10 @@ private:
     RefPtr<ProcessTracer> m_tracer;
     OwnPtr<ELFLoader> m_elf_loader;
 
+    RefPtr<Region> m_master_tls_region;
+    size_t m_master_tls_size { 0 };
+    size_t m_master_tls_alignment { 0 };
+
     Lock m_big_lock { "Process" };
 
     u64 m_alarm_deadline { 0 };