Note: this is preliminary work for helping integration of JEP-331. It is still possible that JEP-331 is not going in and therefore this might be not needed.

In preparation of trying to get JEP-331 integrated, there is a change to the ThreadLocalAllocationBuffer that has a ripple effect across various architecture specific files, each being a line or two.

This item is to rename the field separately, greatly reducing the number of files change in the bigger JEP-331 webrev.

The change also might be considered as a better renaming, as end and hard_end are harder to understand then current_end and reserved_end.

Once the JEP-331 is integrated, there would be a new field called allocation_end. At that point the three end pointers represent:

- fast_path_end : the current end of the TLAB as far as everyone is concerned when using a fast path for the tlab allocation but could be a sampling point if the sampler is enabled
- allocation_end: the space used for allocations of the TLAB (ie. the current _end and proposed _current_end)
- hard_end (via a method): what was actually reserved for the TLAB including some extra space for alignment (which is allocation_end + some space)