RFR: 8254364: Remove leading _ from struct/union declarations in jvmti.h

[Ioi Lam]

This PR changes declarations in jvmti.h like the following from

struct _jvmtiTimerInfo;
typedef struct _jvmtiTimerInfo jvmtiTimerInfo;

to

struct jvmtiTimerInfo;
typedef struct jvmtiTimerInfo jvmtiTimerInfo;

This way, it becomes possible to make forward declaration in C++ code like this, without assuming the knowledge of the
`struct _jvmtiTimerInfo` type:

struct jvmtiTimerInfo;

Please see bug report [JDK-8254364](https://bugs.openjdk.java.net/browse/JDK-8254364) for the before/after versions of
jvmti.h, which is generated from XML.

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Commit messages:
 - 8254364: Remove leading _ from struct/union declarations in jvmti.h

Changes: https://git.openjdk.java.net/jdk/pull/596/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=596&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254364
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/596.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/596/head:pull/596

PR: https://git.openjdk.java.net/jdk/pull/596