Fix freertos_kernel cmake property, Posix Port (#640)

* Fix freertos_kernel cmake property, Posix Port

* Moves the `set_property()` call below the target definition in top level CMakeLists file
* Corrects billion value from `ULL` suffix (not C90 compliant) to `UL` suffix with cast to uint64_t

* Add blank line to CMakeLists.txt

portable/ThirdParty/GCC/Posix/port.c

@@ -343,7 +343,7 @@ static uint64_t prvGetTimeNs( void )
 
     clock_gettime( CLOCK_MONOTONIC, &t );
 
-    return ( uint64_t )t.tv_sec * 1000000000ULL + ( uint64_t )t.tv_nsec;
+    return ( uint64_t )t.tv_sec * ( uint64_t )1000000000UL + ( uint64_t )t.tv_nsec;
 }
 
 static uint64_t prvStartTimeNs;