Support API versioning in platform-specific headers (see #3836)

- Exclude platform-specific includes (anything in < >) from the
  clang preprocessor by using `!defined(GENERATING_CEF_API_HASH)`
  in CEF header files.
- Define "target platforms" by passing platform- and architecture-
  specific ifdefs to the clang preprocessor. Grep for `defined(OS_`
  to identify headers that require target platform processing, and
  then process for each target as the platform-specific API hash
  contribution.
- Delete the univeral hash which is no longer a useful concept.

include/internal/cef_thread_internal.h

@@ -31,12 +31,14 @@
 #define CEF_INCLUDE_INTERNAL_CEF_THREAD_INTERNAL_H_
 #pragma once
 
+#if !defined(GENERATING_CEF_API_HASH)
 #if defined(OS_WIN)
 #include <windows.h>
 #elif defined(OS_POSIX)
 #include <pthread.h>
 #include <unistd.h>
 #endif
+#endif
 
 #include "include/internal/cef_export.h"