Remove excessive inclusion of jvmti.h and jvmtiExport.hpp

jvmti.h is included 905 times and jvmtiExport.hpp is included 776 times (by 971 hotspot .o files). Most of these are unnecessarily included by the following 3 popular header files:

javaClasses.hpp: ThreadStatus is rarely used, and should be moved to a separate header file. The enum is also changed to an enum class for better type safety (see JDK-8247938).

os.hpp: No need to include jvmti.h. Use forward declaration for "struct jvmtiTimerInfo;" instead.

thread.hpp: No need to include jvmExport.hpp. Use forward declaration for JvmtiSampledObjectAllocEventCollector and JvmtiVMObjectAllocEventCollector instead.