test _MSC_VER in SkCpu

We're looking to use tools provided by the compiler toolchain,
not by the OS, so the proper check is for MSVC (or compatible
clang-cl) not for Windows.  E.g. MinGW wants the #else path.

Change-Id: Iefc43a857a1f6438c9145d7503a42c0646c5cbf1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/270096
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

Author: Mike Klein

src/core/SkCpu.cpp

@@ -11,7 +11,7 @@
 #include "src/core/SkCpu.h"
 
 #if defined(SK_CPU_X86)
-    #if defined(SK_BUILD_FOR_WIN)
+    #if defined(_MSC_VER)
         #include <intrin.h>
         static void cpuid (uint32_t abcd[4]) { __cpuid  ((int*)abcd, 1);    }
         static void cpuid7(uint32_t abcd[4]) { __cpuidex((int*)abcd, 7, 0); }