Fix thunk return value in case of int64 or double for Win64

Note:
Win64 X64 uses registry rax for 1,2,4,8 bytes long int or struct return value
it uses xmm0 for float (single) or double floating point values
it uses a pointer passed as hidden first parameter (rcx) for anything else returned by value and copy the pointer to rax.

To pass a double back we use a call to a function taking double & returning double (fakeReturnDouble)
Any of these actions (passing 1 double or returning 1 double) would load xmm0, and we're done.
Except the function is no-op and could be entirely eliminated by aggressive optimizer.
It's thus vital to lay the function into another file and avoid any sort of link time optimization.

TO DO: This code currently does nothing for passing a short struct back by value in rax...
If the structure is long or odd sized, its address is expected as (hidden) first parameter (rcx), rather than on stack.
So the code does not look quite right yet wrt Win64 X64 ABI.

TO DO: the same kind of changes should be applied to X64 system V ABI

platforms/Cross/plugins/IA32ABI/x64win64ia32abicc.c

@@ -234,29 +234,11 @@ extern void saveFloatRegsWin64(long long xmm0,long long xmm1,long long xmm2, lon
 
 	case retword:	return vmcc.rvs.valword;
 
-	case retword64: {
-		long vhigh = vmcc.rvs.valleint64.high;
-#if _MSC_VER
-				_asm mov edx, dword ptr vhigh;
-#elif __GNUC__
-				asm("mov %0,%%edx" : : "m"(vhigh));
-#else
-# error need to load edx with vmcc.rvs.valleint64.high on this compiler
-#endif
-				return vmcc.rvs.valleint64.low;
-	}
+	case retword64:  return (((unsigned long long)vmcc.rvs.valleint64.high) << 32)  | (unsigned int)vmcc.rvs.valleint64.low;
 
-	case retdouble: {
-		double valflt64 = vmcc.rvs.valflt64;
-#if _MSC_VER
-				_asm fld qword ptr valflt64;
-#elif __GNUC__
-				asm("fldl %0" : : "m"(valflt64));
-#else
-# error need to load %f0 with vmcc.rvs.valflt64 on this compiler
-#endif
-				return 0;
-	}
+	case retdouble:
+					fakeReturnDouble( vmcc.rvs.valflt64 );
+					return 0;
 
 	case retstruct:	memcpy( (void *)(stackp[1]),
 							vmcc.rvs.valstruct.addr,

platforms/Cross/plugins/IA32ABI/x64win64stub.c

@@ -14,5 +14,9 @@ void saveFloatRegsWin64(double xmm0,double xmm1,double xmm2, double xmm3,double
 	fpargs[2]=xmm2;
 	fpargs[3]=xmm3;
 }
+double fakeReturnDouble(double xmm0)
+{
+	return xmm0;
+}
 #endif
 #endif