Enable +isomalloc_sync by default for AMPI
If running an AMPI application, it's often necessary to pass the runtime option '+isomalloc_sync' to ensure correctness in the face of Address Space Layout Randomization (ASLR).
The current implementation of +isomalloc_sync requires support for a shared file system across nodes, which is common but not universal. It would be nicer if we enabled +isomalloc_sync runtime support by default (on certain builds/systems only?) and added an option to disable it.