MdePkg/BaseSynchronizationLib GCC: fix X64 InternalSyncCompareExchange64()
    
    (This patch is identical to the X64 half of the last one, except for the
    InternalSyncCompareExchange32() -> InternalSyncCompareExchange64() and
    "cmpxchgl" -> "cmpxchgq" replacements.)
    
    The CMPXCHG instruction has the following operands:
    - AX (implicit, CompareValue):    input and output
    - destination operand (*Value):   input and output
    - source operand (ExchangeValue): input
    
    The X64 version of InternalSyncCompareExchange64() attempts to mark both
    CompareValue and (*Value) as input/output, but it doesn't use the
    appropriate constraints for either operand.
    
    Fix these issues. Furthermore, prefer the short "+" constraint for I/O
    operands over the <output-operand-number> constraint that can be applied
    to the input instances of I/O operands.
    
    Cc: Liming Gao <liming.gao@intel.com>
    Cc: Michael D Kinney <michael.d.kinney@intel.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1208
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>