Fix a typo in checks for MSVC (#8096)

MSVC's defs are different in C++ and CMake :)

Author: dimula73

libdevice/cmath_wrapper_fp64.cpp

@@ -137,7 +137,7 @@ double atanh(double x) { return __devicelib_atanh(x); }
 DEVICE_EXTERN_C_INLINE
 double scalbn(double x, int exp) { return __devicelib_scalbn(x, exp); }
 
-#if defined(MSVC)
+#if defined(_MSC_VER)
 #include <math.h>
 // FLOAT PROPERTIES
 #define _D0 3 // little-endian, small long doubles

sycl/include/sycl/detail/iostream_proxy.hpp

@@ -9,7 +9,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 namespace std {
 #endif
 
-#if defined(MSVC) && defined(_MT) && defined(_DLL)
+#if defined(_MSC_VER) && defined(_MT) && defined(_DLL)
 #define __SYCL_EXTERN_STREAM_ATTRS __declspec(dllimport)
 #else
 #define __SYCL_EXTERN_STREAM_ATTRS

sycl/include/sycl/detail/pi.hpp

@@ -62,7 +62,7 @@ enum TraceLevel {
 bool trace(TraceLevel level);
 
 #ifdef __SYCL_RT_OS_WINDOWS
-#ifdef MSVC
+#ifdef _MSC_VER
 #define __SYCL_OPENCL_PLUGIN_NAME "pi_opencl.dll"
 #define __SYCL_LEVEL_ZERO_PLUGIN_NAME "pi_level_zero.dll"
 #define __SYCL_CUDA_PLUGIN_NAME "pi_cuda.dll"