RFR: 8282306: os::is_first_C_frame(frame*) crashes on invalid link access [v6]

[Johannes Bechberger]

> This PR introduces a new method `can_access_link` into the frame class to check the accessibility of the link information. It furthermore adds a new `os::is_first_C_frame(frame*, Thread*)` that uses the `can_access_link` method
> and the passed thread object to check the validity of frame pointer, stack pointer, sender frame pointer and sender stack pointer. This should reduce the possibilities for crashes.

Johannes Bechberger has updated the pull request incrementally with two additional commits since the last revision:

 - Simple test
 - Use os::is_readable_pointer

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7591/files
  - new: https://git.openjdk.java.net/jdk/pull/7591/files/e91bfeef..2d29a6db

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7591&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7591&range=04-05

  Stats: 75 lines in 5 files changed: 63 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7591.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7591/head:pull/7591

PR: https://git.openjdk.java.net/jdk/pull/7591