RFR: 8294924: JvmtiExport::post_exception_throw() doesn't deal well with concurrent stack processing

[Erik Österlund]

There is a stack walk in JvmtiExport::post_exception_throw() that has safepoints in it. This trips up the stack watermark code. This patch adds a RAII object to JvmtiExport::post_exception_throw() that keeps the thread and its stack fully processed throughout the function.
Testing: tier1-7 of ZGC tests on linux x86_64 debug and manual testing of the test that failed.

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Commit messages:
 - Remove trailing whitespace
 - 8294924: JvmtiExport::post_exception_throw() doesn't deal well with concurrent stack processing

Changes: https://git.openjdk.org/jdk/pull/11238/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11238&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294924
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11238.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11238/head:pull/11238

PR: https://git.openjdk.org/jdk/pull/11238