RFR: 8263392: Allow current thread to be specified in ExceptionMark

[Ioi Lam]

`ExceptionMark`, usually used via the `EXCEPTION_MARK` marco, is used to guarantee that an exception is not thrown within a block of code. I made two changes to improve efficiency:

- Avoid calling `Thread::current()` if a thread object is already available.
- Avoid passing a reference to the `ExceptionMark` constructor. This helps C++ generate slightly better code.

This new variant of `ExceptionMark` is mainly intended for future clean up of `TRAPS/CHECK/THREAD` code, where an exception context is temporarily needed but we will guarantee that all exceptions will be handled. I modified `SharedRuntime::monitor_exit_helper()` to use this pattern:

Old style:

void a_func_that_never_throws() {
  EXCEPTION_MARK;
  a_func_that_could_throw(THREAD);
  if (HAS_PENDING_EXCEPTION) {
      // handle it
    CLEAR_PENDING_EXCEPTION;
  }
}

New style:

void a_func_that_never_throws(Thread* current) { // pass thread to avoid calling Thread::current()
  ExceptionMark em(current);
  Thread* THREAD = current; // For exception macros.
  a_func_that_could_throw(THREAD);
  if (HAS_PENDING_EXCEPTION) {
      // handle it
      CLEAR_PENDING_EXCEPTION;
  }
}

-------------

Commit messages:
 - fixed build
 - 8263392: Allow current thread to be specified in ExceptionMark

Changes: https://git.openjdk.java.net/jdk/pull/2950/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2950&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263392
  Stats: 35 lines in 8 files changed: 17 ins; 3 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2950.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2950/head:pull/2950

PR: https://git.openjdk.java.net/jdk/pull/2950