MdePkg/BaseSynchronizationLib GCC: fix InternalSyncCompareExchange16()

The CMPXCHG instruction has the following operands:
- AX (implicit, CompareValue):    input and output
- destination operand (*Value):   input and output
- source operand (ExchangeValue): input

The IA32 version of InternalSyncCompareExchange16() correctly marks
CompareValue as input/output, but it marks (*Value) only as input.

The X64 version of InternalSyncCompareExchange16() 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>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/BaseSynchronizationLib/Ia32/GccInline.c

@@ -115,11 +115,10 @@ InternalSyncCompareExchange16 (
 {
   __asm__ __volatile__ (
     "lock                 \n\t"
-    "cmpxchgw    %1, %2   \n\t"
-    : "=a" (CompareValue)       // %0
-    : "q"  (ExchangeValue),     // %1
-      "m"  (*Value),            // %2
-      "0"  (CompareValue)       // %3
+    "cmpxchgw    %2, %1   \n\t"
+    : "+a" (CompareValue),      // %0
+      "+m" (*Value)             // %1
+    : "q"  (ExchangeValue)      // %2
     : "memory",
       "cc"
     );

MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c

@@ -115,12 +115,10 @@ InternalSyncCompareExchange16 (
 {
   __asm__ __volatile__ (
     "lock                 \n\t"
-    "cmpxchgw    %3, %1   \n\t"
-    : "=a" (CompareValue),      // %0
-      "=m" (*Value)             // %1
-    : "a"  (CompareValue),      // %2
-      "r"  (ExchangeValue),     // %3
-      "m"  (*Value)             // %4
+    "cmpxchgw    %2, %1   \n\t"
+    : "+a" (CompareValue),      // %0
+      "+m" (*Value)             // %1
+    : "r"  (ExchangeValue)      // %2
     : "memory",
       "cc"
     );