Mützel noted that the bytecode interpreter testsuite ran suspiciously slow on a Alpine 32-bit buildbot.

https://octave.discourse.group/t/wip-stack-vm-for-octave/2884/176

This patch changes the stack element size on 32-bit systems to 32-bit instead of 64-bit, which should speed 32-bit platforms up a bit.

It could not recreate the issue however on my Debian 32-bit VM. 'make check' and 'make check-vm'
runs in about the same time (without and with the patch). Running through a VM seem to introduce too much noise to benchmark the patch too, but smaller stack elements should be fundamentally better.

However 'make check' and 'make check-vm' choked on 'quadcc.cc-tst' with or with-out the patch, so I had to delete it to run the testsuite. The testsuite takes 5 minutes without quadcc.cc-tst and with it, it didn't complete in an hour.

Remove always 64-bit type fields in stack element union, to decrease
the memory footprint of the stack. Instead use octave_idx_type that
is 32-bits on 32-bit systems.

* pt-bytecode.h: (stack_element) Remove always 64-bit types.
                                 Add octave_idx_type idx field instead.
* pt-bytecode-vm.cc: Use idx field where appropiate.