unwind: EHABISectionIterator `operator!=`, constify `operator-`

Add missing `operator!=` and make `operator-` const for
`EHABISectionIterator`.  This repairs the build of libunwind when
building with GCC.

Patch by Chad Duffin!

Reviewed By: compnerd, libunwind
Differential Revision: https://reviews.llvm.org/D81597