RFR: 8297967: Make frame::safe_for_sender safer

[Johannes Bechberger]

Makes frame::safe_for_sender safer by using os::is_readable_pointer to check the location of the return address.

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Commit messages:
 - 8297967: Make frame::safe_for_sender safer

Changes: https://git.openjdk.org/jdk/pull/11461/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11461&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297967
  Stats: 14 lines in 3 files changed: 10 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11461.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11461/head:pull/11461

PR: https://git.openjdk.org/jdk/pull/11461