pahole: Optimize Hotspot C++ class layouts

Umbrella task to look through the pahole report for libjvm.so and see which classes need a bit of field reorganization to optimize footprint. The bar should be: the class should be instantiated often enough to matter, and the projected savings should be >=8 bytes, which would provide enough saving on 64-bit platforms not to to be eaten with object alignment.

Some of this work was done before in JDK-8006280.

A good starting point would be going over the output of:

```
pahole -MP shipilev-jdk/build/linux-aarch64-server-release/images/jdk/lib/server/libjvm.so
```

...and then looking at concrete classes with

```
pahole -M shipilev-jdk/build/linux-aarch64-server-release/images/jdk/lib/server/libjvm.so
```

The actual work should be done in subtasks, since it would naturally be grouped by component/area/cause.