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

[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

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

Commit messages:
 - only use executable memory if supported

Changes: https://git.openjdk.java.net/jdk/pull/6960/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6960&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267341
  Stats: 21 lines in 1 file changed: 17 ins; 4 del; 0 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