ExecMem is defined in os.hpp as:

// Executable parameter flag for os::commit_memory() and
// os::commit_memory_or_exit().
const bool ExecMem = true;

and only ever used as "!ExecMem" in calls to os::commit_memory().

Maybe it did something in the past. Now it just seems to be a complicated way to say "false". So we should say "false".