RFR: 8252526: Remove excessive inclusion of jvmti.h and jvmtiExport.hpp

[Ioi Lam]

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

* javaClasses.hpp: `java_lang_Class::ThreadStatus` (which depends on jvmti.h) this type is rarely used. Move this type to a separate header file. The enum is also changed to an enum class for better type safety.
* 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.

Tested with mach5 tier1-2 and build tiers 3-5.

Note: Many files are changed, but most of the changes are adding a missing jvmtiExports.hpp.

Original review thread: https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-August/041509.html

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Commit messages:
 - 8252526: Remove excessive inclusion of jvmti.h and jvmtiExport.hpp

Changes: https://git.openjdk.java.net/jdk/pull/1152/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1152&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252526
  Stats: 232 lines in 65 files changed: 125 ins; 34 del; 73 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1152.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1152/head:pull/1152

PR: https://git.openjdk.java.net/jdk/pull/1152