ntdll: Allow getting/setting x86_64 context of x86 processes in wine64.

ntdll: Allow getting/setting x86_64 context of x86 processes in wine64.

WoW64 process has two separate contexts:
- x86 context used most of the time (e.g. by application code)
- x86_64 context used by system when it quits x86 emulation and jumps to
  the kernel code
A notable exception are debug registers - their state is shared. Some
debuggers make use of that fact and sets/gets debug registers of x86
processes using x86_64 thread context.

Add support for setting and getting debug registers using x86_64
thread context. Getting other registers is allowed too and will return
values from x86 thread context.

Fixes hardware breakpoints in IDA 7.0 disassembler (x86_64 app) when
debugging x86 (32 bit) applications.

Signed-off-by: Rafał Harabień <rafalh92@outlook.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>