RFR: 8267341: macos attempt_reserve_memory_at(arg1, arg2, true) failure [v2]

[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 Dan's feedback

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6960/files
  - new: https://git.openjdk.java.net/jdk/pull/6960/files/202a636f..4e06e7bd

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6960&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6960&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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