Merge the x86 gdt function and constant definitions

Bump MAX_USERLDT_SIZE to the max size (wastes some memory). wine needs more
than PAGE_SIZE and fails spuriously.
XXX: Note the duplicate definition hacks. Should really create <x86/gdt.h>,
put the just the constants there and unify them.
This would also avoid the hack in: src/tests/lib/libi386/t_user_ldt.c#46

branches: 1.12.6;
Merge the bouyer-xenpvh branch, bringing in Xen PV drivers support under HVM
guests in GENERIC.
Xen support can be disabled at runtime with
boot -c
disable hypervisor

Give the ldt a fixed size of one page (512 slots), and drop the variable-
sized mechanism that was too complex.

This fixes a race between USER_LDT and SVS: during context switches, the
way SVS installs the new ldt relies on the ldt pointer AND the ldt size,
but both cannot be accessed atomically at the same time.

branches: 1.10.24;
Remove gdt_reload_cpu. GDTR takes a VA as base, and in our x86
implementation this VA is per-cpu and does not change; there is therefore
no need to remotely reload GDTR.

Localify, add a comment and merge some others.

branches: 1.8.2;
Make this area compile, even if we don't support USER_LDT on amd64.

branches: 1.7.18; 1.7.36; 1.7.40;
add the guts of TLS support on amd64. based on joerg's patch,
reworked by me to support 32-bit processes as well.
we now keep %fs and %gs loaded with the user values
while in the kernel, which means we don't need to
reload them when returning to user mode.

branches: 1.6.2; 1.6.4;
Remove all the __P() from sys (excluding sys/dist)
Diff checked with grep and MK1 eyeball.
i386 and amd64 GENERIC and sys still build.

branches: 1.5.8; 1.5.14;
Remove clause 3 and 4 from TNF licenses

branches: 1.4.6; 1.4.8; 1.4.10;
- make amd64 use per-cpu tss.
- fix iopl syscall for amd64+xen.

branches: 1.3.50; 1.3.56; 1.3.64;
merge ktrace-lwp.

branches: 1.2.12;
When converting GDT length units from segment structures to bytes for the
amd64 port, I converted MINGDTSIZ wrongly; it was not page aligned, causing
gdt_grow to corrupt the GDT. Fix this, and remove the extraneous definitions
of the sizes from gdt.c.

From OpenBSD.

branches: 1.1.2; 1.1.4;
Rename the x86_64 port to amd64, as this is the actual name used for
the processor family now. x86_64 is kept as the MACHINE_ARCH value,
since it's already widely used (by e.g. the toolchain, etc), and
by other operating systems.