ET_HAS_EXCEPTIONS: require defined(_MSC_VER) to conclude _HAS_EXCEPTIONS implies exceptions are on

_HAS_EXCEPTIONS might get defined on some other platform by coincidence; require it to be actual MSVC for us to conclude exceptions are on.

Differential Revision: [D76857541](https://our.internmc.facebook.com/intern/diff/D76857541/)

[ghstack-poisoned]

Author: swolchok

runtime/platform/compiler.h

@@ -173,7 +173,7 @@ using ssize_t = ptrdiff_t;
 
 #ifdef __EXCEPTIONS
 #define ET_HAS_EXCEPTIONS 1
-#elif defined(_HAS_EXCEPTIONS) && _HAS_EXCEPTIONS
+#elif defined(_MSC_VER) && defined(_HAS_EXCEPTIONS) && _HAS_EXCEPTIONS
 #define ET_HAS_EXCEPTIONS 1
 #else
 #define ET_HAS_EXCEPTIONS 0