MdeModulePkg/DebugSupportDxe: Fix XCODE5 build failure

MdeModulePkg/DebugSupportDxe: Fix XCODE5 build failure

https://bugzilla.tianocore.org/show_bug.cgi?id=572

Update X64 AsmFuncs.S to resolve a build failure using the
XCODE5 tool chain.  This change updates AsmFuncs.S to match
AsmFuncs.asm and AsmFuncs.nasm.

The error generated in XCODE5 build is:

error: invalid instruction mnemonic 'movzxw'
    movzxw 8(%rax), %rax
    ^~~~~~

The correct instruction is

    movzwq 8(%rax), %rax

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Andrew Fish <afish@apple.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Andrew Fish <afish@apple.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>