kernel/x86_64: adjust descriptors tables for compatibility mode.

* also adjust BOOT_GDT_SEGMENT_COUNT for x86, the definition is used by the
boot loader.
* add some 32-bit definitions.
* add a UserTLSDescriptor class, this will be used by 32-bit threads.

Change-Id: I5b1d978969a1ce97091a16c9ec2ad7c0ca831656

haiku_loader: Fix wrong size of gBootGDT on x86_64.

The BOOT_GDT_SEGMENT_COUNT was based on USER_DATA_SEGMENT on both
x86 and x86_64. However, on x86_64 the order of the segments is
different, leading to a too small gBootGDT array. Move the define to
the arch specific headers so they can be setup correctly in either case.
Also add a STATIC_ASSERT() to check that the descriptors fit into the
array.

Pointed out by CID 1210898.

x86: Store pointer to the current thread in gs:0

Apparently, reading from dr3 is slower than reading from memory
with cache hit.

Also, depending on hypervisor configuration, accessing dr3 may cause
a VM exit (and, at least on kvm, it does), what makes it much slower
than a memory access even when there is a cache miss.

x86: Let each CPU have its own GDT

Cleaned up 32-bit descriptors.h.

Support for SMP on x86_64.

No major changes to the kernel: just compiled in arch_smp.cpp and fixed the
IDT load in arch_cpu_init_percpu to use the correct limit for x86_64 (uses
sizeof(interrupt_descriptor)). In the boot loader, changed smp_boot_other_cpus
to construct a temporary GDT and get the page directory address from CR3, as
what's in kernel_args will be 64-bit stuff and will not work to switch the
CPUs into 32-bit mode in the trampoline code. Refactored 64-bit kernel entry
code to not use the stack after disabling paging, as the secondary CPUs are
given a 32-bit virtual stack address by the SMP trampoline code which will
no longer work.

Added arch_cpu.cpp to the x86_64 build.

* Some things are currently ifndef'd out completely for x86_64 because
they aren't implemented, there's a few other ifdef's to handle x86_64
differences but most of the code works unchanged.
* Renamed some i386_* functions to x86_*.
* Added a temporary method for setting the current thread on x86_64
(a global variable, not SMP safe). This will be changed to be done
via the GS segment but I've not implemented that yet.

Merged x86_64 headers into x86 headers.

Not many changes seeing as there's not much x86_64 stuff done yet. Small
differences are handled with ifdefs, large differences (descriptors.h,
struct iframe) have separate headers under arch/x86/32 and arch/x86/64.

haiku_loader: Fix wrong size of gBootGDT on x86_64.

The BOOT_GDT_SEGMENT_COUNT was based on USER_DATA_SEGMENT on both
x86 and x86_64. However, on x86_64 the order of the segments is
different, leading to a too small gBootGDT array. Move the define to
the arch specific headers so they can be setup correctly in either case.
Also add a STATIC_ASSERT() to check that the descriptors fit into the
array.

Pointed out by CID 1210898.

x86: Store pointer to the current thread in gs:0

Apparently, reading from dr3 is slower than reading from memory
with cache hit.

Also, depending on hypervisor configuration, accessing dr3 may cause
a VM exit (and, at least on kvm, it does), what makes it much slower
than a memory access even when there is a cache miss.

x86: Let each CPU have its own GDT

Cleaned up 32-bit descriptors.h.

Support for SMP on x86_64.

No major changes to the kernel: just compiled in arch_smp.cpp and fixed the
IDT load in arch_cpu_init_percpu to use the correct limit for x86_64 (uses
sizeof(interrupt_descriptor)). In the boot loader, changed smp_boot_other_cpus
to construct a temporary GDT and get the page directory address from CR3, as
what's in kernel_args will be 64-bit stuff and will not work to switch the
CPUs into 32-bit mode in the trampoline code. Refactored 64-bit kernel entry
code to not use the stack after disabling paging, as the secondary CPUs are
given a 32-bit virtual stack address by the SMP trampoline code which will
no longer work.

Added arch_cpu.cpp to the x86_64 build.

* Some things are currently ifndef'd out completely for x86_64 because
they aren't implemented, there's a few other ifdef's to handle x86_64
differences but most of the code works unchanged.
* Renamed some i386_* functions to x86_*.
* Added a temporary method for setting the current thread on x86_64
(a global variable, not SMP safe). This will be changed to be done
via the GS segment but I've not implemented that yet.

Merged x86_64 headers into x86 headers.

Not many changes seeing as there's not much x86_64 stuff done yet. Small
differences are handled with ifdefs, large differences (descriptors.h,
struct iframe) have separate headers under arch/x86/32 and arch/x86/64.