RFR (S) 8217994: os::print_hex_dump should be more resilient against unreadable memory

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8217994

Fix:
  http://cr.openjdk.java.net/~shade/8217994/webrev.01/

This is related to JDK-8217879 (hs_err should print more instructions in hex dump), and this more
generic fix should cover more cases in error handler. New gtest verifies we can call os::p_h_d on
bad memory now. It also implicitly verifies that SafeFetch machinery works fine. Consider running
that gtest (make images run-test TEST=gtest:os) on your platform if you suspect it does not.

Testing: (Linux, Windows) x86_64 build, gtest, eyeballing gtest output

Thanks,
-Aleksey