RFR: 8267341: macos attempt_reserve_memory_at(arg1, arg2, true) failure [v3]

[Gerard Ziemski]

> Re-enable **release_multi_mappings** gtest on macOS
> 
> This test would occasionally fail on macOS, but thanks to Dan's catch, it turned out that it actually only fails on those test machines that run macOS 10.13.6 or earlier.
> 
> The proposed fix simply makes a single call to `os::reserve_memory()` with the `executable` flag `True`, and if that failed forces `executable`  to `False` later in the actual test code (alternatively we could have just also skipped that test portion completely, but this way we actually do something rather than nothing at all).
> 
> Tested manually on macOS 10.13.6 and via Mach5

Gerard Ziemski has updated the pull request incrementally with one additional commit since the last revision:

  incorporate David's feedback by factoring out the workaround check code into separate function

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6960/files
  - new: https://git.openjdk.java.net/jdk/pull/6960/files/4e06e7bd..f4bf7eba

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6960&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6960&range=01-02

  Stats: 22 lines in 1 file changed: 13 ins; 6 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6960.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6960/head:pull/6960

PR: https://git.openjdk.java.net/jdk/pull/6960