Remove julong types in os::limit_heap_by_allocatable_memory

In JDK-8257901 the request came up to change the julong types (return value, arguments) in os::limit_heap_by_allocatable_memory to size_t.

We shouldn't use the j-types if not absolutely necessary in VM code, and in this instance size_t seems to be a perfect fit.

This has been deferred due to that propagating a (small?) bit through some methods in the OS class.

Implement.