[CMAKE] msvc.cmake: Remove add_definitions(/D__arm__) (reactos#3710)

Author: SergeGautherie

sdk/cmake/msvc.cmake

@@ -363,7 +363,6 @@ if(ARCH STREQUAL "amd64")
     add_definitions(/D__x86_64)
     set(SPEC2DEF_ARCH x86_64)
 elseif(ARCH STREQUAL "arm")
-    add_definitions(/D__arm__)
     set(SPEC2DEF_ARCH arm)
 elseif(ARCH STREQUAL "arm64")
     add_definitions(/D__arm64__)