MdeModulePkg DebugSupportDxe: Fix build error with GNU assembler

The GNU assembler (2.24.51.20140918) is failing to build when movw is used on this instruction. 
Instead use the mov instruction, which matches the other cases of loading segment registers.

The error message seen is:
  AsmFuncs.iii:283: Error: incorrect register `%rax' used with `w' suffix

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@16529 6f19259b-4bc3-4df7-8a09-765794883524