fix(win): oiiotool --buildinfo misreported platform on MSVS (AcademySoftwareFoundation#5027)

Need to test some MSVS-specific macros to determine what architecture to
report.

And especially, if it doesn't know the processor architecture, it still
should be *appending* that to the platform, not replacing it! This
caused MSVS-compiled OIIO on Windows to report "unknown arch?"

Signed-off-by: Larry Gritz <lg@larrygritz.com>
Signed-off-by: Vlad (Kuzmin) Erium <libalias@gmail.com>

Author: lgritz

src/libOpenImageIO/imageio.cpp

@@ -258,14 +258,15 @@ oiio_build_platform()
     platform = "UnknownOS";
 #endif
     platform += "/";
-#if defined(__x86_64__)
+#if defined(__x86_64__) || defined(_M_AMD64)
     platform += "x86_64";
-#elif defined(__i386__)
+#elif defined(__i386__) || defined(_M_IX86)
     platform += "i386";
-#elif defined(_M_ARM64) || defined(__aarch64__) || defined(__aarch64)
+#elif defined(_M_ARM64) || defined(__aarch64__) || defined(__aarch64) \
+    || defined(__ARM_ARCH)
     platform += "ARM";
 #else
-    platform = "unknown arch?";
+    platform += "unknown arch?";
 #endif
     return platform;
 }