MdePkg/BaseLib: Add stack switch related definitions for IA32

The new definitions include two structures

  IA32_TASK_STATE_SEGMENT
  IA32_TSS_DESCRIPTOR

two macros

  IA32_GDT_TYPE_TSS
  IA32_GDT_ALIGNMENT

and one API

  VOID
  EFIAPI
  AsmWriteTr (
    IN UINT16 Selector
    );

They're needed to setup task gate and interrupt stack table for stack switch.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Suggested-by: Ayellet Wolman <ayellet.wolman@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jeff Fan <vanjeff_919@hotmail.com>
Reviewed-by: Jiewen.yao@intel.com

MdePkg/Library/BaseLib/X64/WriteTr.nasm

@@ -0,0 +1,37 @@
+;------------------------------------------------------------------------------ ;
+; Copyright (c) 2017, Intel Corporation. All rights reserved.<BR>
+; This program and the accompanying materials
+; are licensed and made available under the terms and conditions of the BSD License
+; which accompanies this distribution.  The full text of the license may be found at
+; http://opensource.org/licenses/bsd-license.php.
+;
+; THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+; WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+;
+; Module Name:
+;
+;   WriteTr.nasm
+;
+; Abstract:
+;
+;   Write TR register
+;
+; Notes:
+;
+;------------------------------------------------------------------------------
+
+    DEFAULT REL
+    SECTION .text
+
+;------------------------------------------------------------------------------
+; VOID
+; AsmWriteTr (
+;   UINT16 Selector
+;   );
+;------------------------------------------------------------------------------
+global ASM_PFX(AsmWriteTr)
+ASM_PFX(AsmWriteTr):
+    mov     eax, ecx
+    ltr     ax
+    ret
+