The MAP_JIT flag is only supported on macOS 10.14 and higher, so mmap()
may fail on older ones.

Bug: chromium:985816
Change-Id: If0e12d61b7f243d804e0213a43160e7b3db106d3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34511Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>