[interp] Replace frame_objref with a volatile store to local. (mono/mono#16790)

[interp] Replace frame_objref with a volatile store to local. (mono/mono#16790)

* [interp] Replace frame_objref with volatile stores to non-volatile locals.
Non-volatiles subject to volatile read/write still get stack-packed.
Volatiles do not.

This will possibly conserve stack, unless MINT_NEWOBJ_FAST is critical path.
 It looks like it is -- enabling this for Linux/gcc/amd64 grows
  frame from 0x98 to 0xA8.

In either case, it is more efficient as the reads do not have to be volatile.
The value can be on the stack and in registers. The stack value will pin it.

* PR: Comments.

* PR: Rearrange comment for correctness, same length.

Commit migrated from https://github.com/mono/mono/commit/735b2cd20c51a926aae93f50903b4e022e851c58